1 00:00:01,673 --> 00:00:04,775 [narrator] Scientists believ there is a hidden substance deep in space 2 00:00:04,777 --> 00:00:07,111 that keeps the cosmos runnin 3 00:00:08,180 --> 00:00:11,048 But is that substance real? 4 00:00:11,050 --> 00:00:14,385 We've never seen dark matter, it's completely invisible, 5 00:00:14,387 --> 00:00:17,354 but we know that it has to be there. 6 00:00:17,356 --> 00:00:22,459 Not only can you not see it, you couldn't really touch it or taste it, or smell it, 7 00:00:22,528 --> 00:00:25,662 and yet it is all around us it affects everything that we do. 8 00:00:25,731 --> 00:00:27,664 [narrator] After searching for decades, 9 00:00:27,733 --> 00:00:32,136 we still don't understand this inexplicable substance 10 00:00:32,138 --> 00:00:36,840 [de Rham] We know dark matte is there because we feel its strong gravitational pul 11 00:00:36,842 --> 00:00:39,676 but it just doesn't want to talk to us. 12 00:00:39,678 --> 00:00:41,578 [narrator] There's evidence that dark matter 13 00:00:41,580 --> 00:00:45,582 makes up 85% of all the matter in the universe. 14 00:00:45,651 --> 00:00:49,153 We can see dark matter holding galaxies together 15 00:00:49,188 --> 00:00:53,991 and ripping other structures apart, we even see it bending light. 16 00:00:53,993 --> 00:00:58,629 Dark matter itself has been around since the beginning of the universe. 17 00:00:58,631 --> 00:01:02,533 Without dark matter, we wouldn't be here. 18 00:01:02,535 --> 00:01:04,701 [narrator] But if you can't see dark matter 19 00:01:04,703 --> 00:01:09,006 and if you can't touch it, does it really exist? 20 00:01:22,922 --> 00:01:25,489 [narrator] The Hyades star cluster. 21 00:01:26,859 --> 00:01:32,863 This family of 700 stars is 150 light years from Eart 22 00:01:32,865 --> 00:01:36,266 At the scale of the universe it's in our backyard. 23 00:01:37,002 --> 00:01:38,836 Hyades is actually close enough to Earth 24 00:01:38,838 --> 00:01:39,970 that you could see it with your naked eye. 25 00:01:39,972 --> 00:01:42,206 When you look up at the night sky, 26 00:01:42,208 --> 00:01:45,943 Hyades is in that V-shape in Taurus the Bull. 27 00:01:47,613 --> 00:01:50,948 [narrator] For most of its 650-million-year lifetime, 28 00:01:51,016 --> 00:01:54,685 the Hyades enjoyed a peaceful existence. 29 00:01:54,687 --> 00:01:58,055 But something is breaking the calm. 30 00:01:58,123 --> 00:02:01,625 The Hyades cluster is one of the most well-studied clusters of stars 31 00:02:01,627 --> 00:02:03,660 we have in the entire sky and yet there's something 32 00:02:03,729 --> 00:02:06,330 very deeply mysterious going on. 33 00:02:07,366 --> 00:02:11,068 [narrator] Two star tails extend from the cluster center, 34 00:02:11,103 --> 00:02:17,674 they should be roughly equal but one tail is hemorrhaging stars. 35 00:02:17,676 --> 00:02:20,677 Something is disrupting it, there's something exerting a force on it 36 00:02:20,679 --> 00:02:24,114 that's ripping stars out of their orbits. 37 00:02:25,885 --> 00:02:28,752 [narrator] Something with immense gravitational pull 38 00:02:28,820 --> 00:02:30,454 has passed by the cluster 39 00:02:31,257 --> 00:02:33,590 and robbed it of stars. 40 00:02:33,592 --> 00:02:37,895 In order to be gravitationally pulling stars out of an object like Hyades, 41 00:02:37,897 --> 00:02:39,730 you need to have an incredibly massive structure, 42 00:02:39,765 --> 00:02:43,000 as much as 10 million times the mass of the sun. 43 00:02:44,036 --> 00:02:48,005 [narrator] This monstrous cosmic mugger should still be visible, 44 00:02:48,507 --> 00:02:50,607 but when we point our telescopes 45 00:02:50,609 --> 00:02:55,078 to where it should be, that region is empty. 46 00:02:56,115 --> 00:03:00,217 There's nothing there and I mean nothing. 47 00:03:00,219 --> 00:03:04,087 And not a little bit or something dark, or something small, 48 00:03:04,089 --> 00:03:06,223 but there's literally nothing that we can see. 49 00:03:07,793 --> 00:03:12,763 [narrator] We know somethin is out there, invisible and powerful. 50 00:03:12,765 --> 00:03:15,899 And whenever we witness these unseen assaults, 51 00:03:15,935 --> 00:03:18,835 a prime suspect gets called in, 52 00:03:18,837 --> 00:03:20,304 a phantom of physics, 53 00:03:21,574 --> 00:03:23,307 dark matter. 54 00:03:24,777 --> 00:03:31,415 So what can we confidently s about this mysterious cosmic substance? 55 00:03:31,417 --> 00:03:37,087 It does not emit light, it does not reflect light, it does not absorb light. 56 00:03:37,089 --> 00:03:40,891 The only thing we know about dark matter is that it has gravity. 57 00:03:40,893 --> 00:03:42,526 We're not even really sure it's matter at all. 58 00:03:42,528 --> 00:03:45,295 It's just that that's the only thing we know, that it has gravity. 59 00:03:45,364 --> 00:03:49,366 [narrator] We may not be abl to see or touch dark matter 60 00:03:49,368 --> 00:03:54,671 but we are very good at finding its fingerprints all over the universe. 61 00:03:54,740 --> 00:04:00,043 We can see dark matter's use of gravity to break and bind structures 62 00:04:00,112 --> 00:04:03,480 and we've been spotting its handiwork for decades. 63 00:04:04,850 --> 00:04:08,018 Let's rewind back to 1933. 64 00:04:09,488 --> 00:04:13,957 Swiss-American physicist, Fritz Zwicky tracks strange movements 65 00:04:13,959 --> 00:04:18,462 in a far off collection of galaxies called the Coma cluster. 66 00:04:18,464 --> 00:04:22,933 He knows he's not seeing the whole picture. 67 00:04:22,935 --> 00:04:28,772 Some galaxies are speeding around the cluster at inexplicably fast rates. 68 00:04:28,774 --> 00:04:30,674 [Bullock] Zwicky is looking at these galaxies 69 00:04:30,676 --> 00:04:34,544 and if the only mass that was there were the other galaxies you can see, 70 00:04:34,613 --> 00:04:38,315 you would expect these galaxies to be moving at about 50 miles a second, 71 00:04:38,317 --> 00:04:41,618 then they would stay bound to each other and not fly apart. 72 00:04:41,620 --> 00:04:45,289 Instead, he sees them moving at 1,000 miles a second. 73 00:04:46,725 --> 00:04:48,492 [narrator] At these velocities, 74 00:04:48,494 --> 00:04:53,230 galaxies should be flying of the cluster like sparks from fireworks. 75 00:04:54,933 --> 00:04:59,002 Zwicky realized there had to be extra stuff, 76 00:04:59,338 --> 00:05:03,106 in his words, Dunkle Materie 77 00:05:03,141 --> 00:05:04,908 Dark matter. 78 00:05:04,910 --> 00:05:05,308 Dark matter. 79 00:05:05,310 --> 00:05:07,377 Dark matter. 80 00:05:07,379 --> 00:05:13,817 [narrator] It becomes clear that Zwicky's Coma cluster isn't an isolated case. 81 00:05:13,885 --> 00:05:19,923 Astronomers begin seeing the same dynamics within galaxies themselves. 82 00:05:19,991 --> 00:05:23,126 In systems governed exclusively by gravity, 83 00:05:23,195 --> 00:05:25,295 objects farthest away from the center 84 00:05:25,364 --> 00:05:27,964 would take the longest to complete an orbit. 85 00:05:27,966 --> 00:05:32,502 But in many galaxies, stars on the outside are orbiting 86 00:05:32,504 --> 00:05:36,306 at almost the same rate as those in the core. 87 00:05:36,308 --> 00:05:37,807 [Thaller] It's almost like a photograph record. 88 00:05:37,809 --> 00:05:41,878 Every part of that record spins around like a solid disc. 89 00:05:41,880 --> 00:05:46,416 The stars are going too fast to stay bound to the gravity of the galaxy. 90 00:05:46,418 --> 00:05:49,019 They should just fly right off into space. 91 00:05:49,521 --> 00:05:53,256 [narrator] Physicists come u with an explanation. 92 00:05:53,292 --> 00:05:59,763 Galaxies sit in a giant hal or ball of invisible dark matter. 93 00:05:59,765 --> 00:06:04,101 And it's that extra mass that allows the stars to turn fast 94 00:06:04,103 --> 00:06:06,937 all the way out to the galactic rim. 95 00:06:06,939 --> 00:06:08,638 [Thaller] Think about actual taking a disc of dough 96 00:06:08,640 --> 00:06:10,774 and spinning it to make a pizza. 97 00:06:10,842 --> 00:06:15,479 The more you spin it, the more those outer regions go farther and farther away. 98 00:06:15,481 --> 00:06:17,748 Eventually, the dough just goes flying everywhere, 99 00:06:17,816 --> 00:06:21,184 that's what would happen to a galaxy if it weren't for dark matter. 100 00:06:21,186 --> 00:06:24,054 Uh, as you spin pizza dough and you spin it faster and faster, 101 00:06:24,056 --> 00:06:25,889 it does hold itself together 102 00:06:25,891 --> 00:06:30,527 because there's all this yummy gluten that's acting as a glue. 103 00:06:30,595 --> 00:06:33,997 Dark matter is the gluten of our universe. 104 00:06:34,733 --> 00:06:37,734 [narrator] By calculating the mass needed to bind 105 00:06:37,736 --> 00:06:40,871 those speeding outer stars to the galaxy, 106 00:06:40,939 --> 00:06:43,006 physicists are able to estimate 107 00:06:43,008 --> 00:06:47,177 how much visible matter there is compared to dark matter. 108 00:06:47,179 --> 00:06:49,813 The results are staggering. 109 00:06:49,881 --> 00:06:53,884 [Tegmark] All the stuff we thought existed was just maybe 15% of our universe. 110 00:06:53,886 --> 00:06:56,019 That's like if you go to a restaurant 111 00:06:56,021 --> 00:07:00,624 and leave like the measly 15% tip, you know, that's what we are. 112 00:07:00,626 --> 00:07:02,426 I mean, not even the majority substance. 113 00:07:03,595 --> 00:07:05,495 [narrator] We may not be able to see it, 114 00:07:05,497 --> 00:07:10,066 but dark matter makes up some 85% of all matter. 115 00:07:10,101 --> 00:07:14,638 Wherever we look, we can se its gravity having effects. 116 00:07:14,640 --> 00:07:18,842 It glues galaxies like our Milky Way together 117 00:07:18,910 --> 00:07:25,081 And a close look reveals dark matter can also bend light itself. 118 00:07:26,752 --> 00:07:29,586 It's called gravitational lensing. 119 00:07:29,588 --> 00:07:32,722 A massive object can bend space and time, 120 00:07:32,724 --> 00:07:38,728 and light must follow the curves of that space and time. 121 00:07:38,730 --> 00:07:43,834 [narrator] Gigantic clumps of any matter create a gravitational lens. 122 00:07:43,902 --> 00:07:48,605 Dark matter showed its space-warping power in a trick it played 123 00:07:48,607 --> 00:07:53,310 with a gigantic explosion in a far off galaxy cluster 124 00:07:54,980 --> 00:08:00,016 Supernova Refsdal was first detected in November of 2014 125 00:08:00,986 --> 00:08:04,988 Supernova Refsdal is actually one of my favorite recent results 126 00:08:04,990 --> 00:08:07,424 in all of the astronomical literature. 127 00:08:07,426 --> 00:08:08,992 That result blew me away. 128 00:08:09,027 --> 00:08:12,863 So a star explodes, light is emitted in all directions, 129 00:08:12,865 --> 00:08:15,966 and some of it makes its way towards the Earth. 130 00:08:15,968 --> 00:08:18,702 So far so good. This is very standard. 131 00:08:18,770 --> 00:08:20,237 So the flash appears 132 00:08:20,973 --> 00:08:23,340 and then, another flash appears. 133 00:08:24,476 --> 00:08:28,945 We see it again, and again, and again. 134 00:08:28,947 --> 00:08:32,883 [Mingarelli] We see the explosion go off in four parts of the sky. 135 00:08:32,885 --> 00:08:36,119 And then, a year later, a fifth explosion goes off 136 00:08:36,187 --> 00:08:38,088 in a totally different part of the sky. 137 00:08:38,090 --> 00:08:39,990 What's going on? 138 00:08:39,992 --> 00:08:45,662 [narrator] Analysis proves that these multiple explosio are the same supernova. 139 00:08:45,664 --> 00:08:50,133 -[explosion] -But between this one dying star and our telescope 140 00:08:50,135 --> 00:08:53,403 sits a giant mass of dark matter, 141 00:08:53,405 --> 00:08:56,840 a huge gravitational lens. 142 00:08:56,842 --> 00:09:01,511 What that means is that some of these rays of light will take much longer, 143 00:09:01,580 --> 00:09:05,282 more complicated paths through this region of space time. 144 00:09:05,951 --> 00:09:09,519 [narrator] The dark matter lens turns one supernova 145 00:09:09,588 --> 00:09:14,224 into a fireworks display lasting an entire year. 146 00:09:15,494 --> 00:09:19,763 Dark matter affected the trajectory of light from this supernova so much 147 00:09:19,831 --> 00:09:21,865 that for some of those trajectories, 148 00:09:21,900 --> 00:09:24,467 it added a whole light year, 149 00:09:24,536 --> 00:09:28,438 it took a whole extra year for light to reach us. 150 00:09:29,608 --> 00:09:32,842 [narrator] Something is ver definitely out there 151 00:09:32,911 --> 00:09:35,445 distorting our view of the cosmos. 152 00:09:36,682 --> 00:09:41,117 It's a potent clue that dark matter is real. 153 00:09:41,119 --> 00:09:47,090 Now, new evidence suggests that without it, we might not exist at all. 154 00:09:54,866 --> 00:09:58,268 [narrator] The cosmos is filled with an unseen substance, 155 00:09:58,503 --> 00:10:02,739 its mass even bends starligh 156 00:10:02,774 --> 00:10:09,212 Gravitational lensing sugges dark matter holds our entir universe together. 157 00:10:10,515 --> 00:10:14,651 For decades, this specter of space has haunted us. 158 00:10:14,719 --> 00:10:17,787 We've never been able to pin it down. 159 00:10:17,789 --> 00:10:23,927 In 2021, an international te ran a virtual experiment to try to predict 160 00:10:23,929 --> 00:10:27,631 where dark matter should be by letting computers 161 00:10:27,699 --> 00:10:30,400 map out where we think it lives. 162 00:10:30,468 --> 00:10:33,003 [Bullock] Because we think we know how it behaves, 163 00:10:33,071 --> 00:10:36,506 we can model what it should be doing in supercomputer simulations. 164 00:10:37,676 --> 00:10:42,946 [narrator] The team taught the computer how dark matte bends light, 165 00:10:42,948 --> 00:10:49,986 then applied computational power to 17,000 unexplored galaxies. 166 00:10:50,656 --> 00:10:55,558 The model created a dark matter map. 167 00:10:55,560 --> 00:10:58,728 I think a lot of people, when they imagine the universe on the larger scales, 168 00:10:58,730 --> 00:11:01,031 think it's sort of boring, everything's uniform. 169 00:11:01,033 --> 00:11:03,500 But that's not what we see. 170 00:11:03,502 --> 00:11:06,436 What's amazing is that on the larger scales of the universe, 171 00:11:06,438 --> 00:11:09,105 we see a very particular pattern. 172 00:11:09,107 --> 00:11:12,475 When we zoom out, we see this magnificent structure, 173 00:11:12,477 --> 00:11:15,378 this cosmic web that's created by dark matter. 174 00:11:16,581 --> 00:11:19,282 [narrator] The interweaving tendrils of dark matter stretch 175 00:11:19,284 --> 00:11:23,887 for thousands of light year across the cosmos. 176 00:11:23,922 --> 00:11:30,093 At the junctions where matte is concentrated, we find galaxies form, 177 00:11:30,095 --> 00:11:32,462 illuminating the dark scaffold. 178 00:11:34,066 --> 00:11:36,733 If dark matter exists, scientists believe 179 00:11:36,735 --> 00:11:40,937 it makes up 85% of the matte in the universe, 180 00:11:41,005 --> 00:11:45,575 and also controls the remaining 15% regular matter, 181 00:11:45,643 --> 00:11:49,179 like stars, planets, us. 182 00:11:50,482 --> 00:11:54,250 If they're right, dark matte played a critical role 183 00:11:54,252 --> 00:11:58,388 in actually building the universe we see today. 184 00:12:03,061 --> 00:12:04,494 2021. 185 00:12:04,496 --> 00:12:08,231 Astronomers using the SkyMapper observatory in Australia 186 00:12:08,233 --> 00:12:14,738 trains specialist optics on a dwarf galaxy called Tucana II. 187 00:12:14,740 --> 00:12:20,477 The SkyMapper's filters split up the starlight into a spectrum of wavelengt 188 00:12:20,479 --> 00:12:24,347 revealing some very ancient light. 189 00:12:25,751 --> 00:12:30,186 [Tremblay] One of the best clocks that we can put on the universe 190 00:12:30,255 --> 00:12:32,288 is the progress of chemistry. 191 00:12:32,390 --> 00:12:33,456 Right? 192 00:12:33,458 --> 00:12:37,527 The build-up of more complex elements over time. 193 00:12:37,529 --> 00:12:41,898 Stars are nothing if not factories of chemical complexity. 194 00:12:41,900 --> 00:12:45,602 They slam, uh, particles together and create heavier elements, 195 00:12:45,670 --> 00:12:48,238 right, through a process called fusion. 196 00:12:48,273 --> 00:12:52,475 The later the generation of star, the more chemicall complex it is. 197 00:12:52,477 --> 00:12:56,613 [narrator] Tucana II's spectral signature reveals its stars contain 198 00:12:56,681 --> 00:13:00,517 very few of these heavy complex elements. 199 00:13:00,519 --> 00:13:05,822 A clue that lets astrophysicists calculate the age of the galaxy. 200 00:13:05,890 --> 00:13:10,693 These are very, very old stars from the very early days of the universe 201 00:13:10,695 --> 00:13:15,265 when the gas in the universe was not that chemically complex. 202 00:13:15,967 --> 00:13:19,936 Tucana II might be one of the oldest known structur 203 00:13:20,004 --> 00:13:21,905 that we can see in our local universe. 204 00:13:21,973 --> 00:13:25,341 It could be as old as 13 billion years. 205 00:13:25,343 --> 00:13:28,111 You know, almost as old as the universe itself. 206 00:13:29,848 --> 00:13:33,283 [narrator] This grand old la of a galaxy is a tiny thing 207 00:13:33,318 --> 00:13:35,385 Barely 3,000 stars. 208 00:13:37,088 --> 00:13:39,589 And yet, way out on her galactic rim, 209 00:13:39,657 --> 00:13:43,993 stars hurdle around at breakneck speed. 210 00:13:43,995 --> 00:13:47,497 [Plait] When you look at the mass of this ultra-faint dwarf galaxy, 211 00:13:47,499 --> 00:13:50,700 it only has a few thousand times the mass of the sun. 212 00:13:50,702 --> 00:13:52,268 That's really small. 213 00:13:52,270 --> 00:13:54,904 And at the speed it's moving, it should fly apart. 214 00:13:54,906 --> 00:14:00,009 [narrator] Tucana II doesn' break up because it's glued together, 215 00:14:00,011 --> 00:14:04,581 apparently by an incredible amount of dark matter. 216 00:14:04,583 --> 00:14:06,649 When you look at a galaxy like our Milky Way, 217 00:14:06,651 --> 00:14:10,053 it's about 85% dark matter, which is a lot. 218 00:14:10,055 --> 00:14:14,090 But with Tucana II, it's more like 99%. 219 00:14:14,092 --> 00:14:18,361 [narrator] Tucana II is old among the oldest galaxies in the universe 220 00:14:18,363 --> 00:14:22,365 and it is packed full of dark matter. 221 00:14:22,367 --> 00:14:27,437 Simulations suggest this dark matter played a ke role in shaping Tucana II 222 00:14:27,439 --> 00:14:31,574 and other very early galaxie right from the beginning, 223 00:14:31,576 --> 00:14:38,848 gathering regular matter into clumps and building the first galaxies. 224 00:14:38,850 --> 00:14:41,017 [Thaller] The importance of dark matter really can't be overstated. 225 00:14:41,019 --> 00:14:44,454 It has actually controlled the way matter has evolved 226 00:14:44,522 --> 00:14:45,488 since the beginning of the universe. 227 00:14:45,556 --> 00:14:47,357 It brings matter together. 228 00:14:47,359 --> 00:14:53,096 You need this underlying structure of dark matter to make it all happen. 229 00:14:53,098 --> 00:14:58,468 [narrator] Scientists think that for billions of years as the early universe grew, 230 00:14:58,470 --> 00:15:00,670 dark matter called the shots 231 00:15:00,672 --> 00:15:06,109 Without its gravity, structures like the Milky Wa wouldn't have formed. 232 00:15:07,812 --> 00:15:11,147 We've seen dark matter's light-bending effects. 233 00:15:11,149 --> 00:15:14,450 We've even deduced where it should be. 234 00:15:14,452 --> 00:15:18,454 Dark matter really does appear to exist, 235 00:15:18,456 --> 00:15:22,292 but this evidence is indirect, circumstantial 236 00:15:23,161 --> 00:15:27,497 To get conclusive proof that dark matter exists, 237 00:15:27,499 --> 00:15:29,232 don't we need to find some? 238 00:15:29,668 --> 00:15:33,903 If we could find a lump of dark matter [laughs], 239 00:15:33,972 --> 00:15:37,106 um, that would be one of the greatest discoveries in all of nature, 240 00:15:37,108 --> 00:15:38,541 in all of our history, right? 241 00:15:38,543 --> 00:15:40,042 Because we would understand 242 00:15:40,278 --> 00:15:44,280 one of the most fundamental components for how our universe works. 243 00:15:44,816 --> 00:15:47,016 Dropping the title, they love that. 244 00:15:48,153 --> 00:15:52,221 [narrator] It's time to hun for dark matter itself. 245 00:15:52,223 --> 00:15:56,459 Could it be hiding in the darkest place of all 246 00:15:57,796 --> 00:15:59,429 Black holes. 247 00:16:09,574 --> 00:16:11,874 [narrator] Scientists believ an invisible substance 248 00:16:11,943 --> 00:16:15,878 is pulling the strings in our universe. 249 00:16:15,947 --> 00:16:19,882 But until we see it, sense i perhaps even touch it, 250 00:16:19,951 --> 00:16:22,452 dark matter is just a theory 251 00:16:23,855 --> 00:16:26,022 Sometimes though, ideas dreamed up 252 00:16:26,024 --> 00:16:28,324 by scientists come true 253 00:16:28,827 --> 00:16:31,961 like black holes. 254 00:16:31,963 --> 00:16:35,965 Once the stuff of science fiction and children's nightmares, 255 00:16:35,967 --> 00:16:40,770 black holes today are confirmed reality. 256 00:16:40,772 --> 00:16:43,940 So black holes and dark matter have a ton of similarities, right? 257 00:16:43,942 --> 00:16:46,476 You know, an unseen collection of matter 258 00:16:46,478 --> 00:16:48,878 that creates an enormous gravitational field, check. 259 00:16:48,913 --> 00:16:52,882 It bends light and causes gravitational lensing, check. 260 00:16:52,917 --> 00:16:56,352 Tests the boundaries of known physics, check. 261 00:16:57,622 --> 00:16:59,956 [narrator] It seems crazy to even ask, 262 00:16:59,958 --> 00:17:01,858 but could our search for dark matter 263 00:17:01,860 --> 00:17:06,596 end in an idea more than 100 years old? 264 00:17:06,598 --> 00:17:10,099 Could dark matter be black holes? 265 00:17:11,669 --> 00:17:14,570 Black holes appear when stars explode. 266 00:17:14,572 --> 00:17:17,774 And their remaining mass crunches down into a sphere 267 00:17:17,842 --> 00:17:22,845 so dense even light can't escape its gravity. 268 00:17:22,847 --> 00:17:27,350 But that's where the black hole dark matter theory stumbles. 269 00:17:27,352 --> 00:17:29,619 We know that black holes happen. We know how they form. 270 00:17:29,687 --> 00:17:34,924 And we also know that there's nowhere near enough of them to be dark matter. 271 00:17:34,926 --> 00:17:38,728 [narrator] Not enough stars have lived and died in the history of the univer 272 00:17:38,730 --> 00:17:42,932 to create 85% of the matter in it. 273 00:17:42,967 --> 00:17:46,235 If dark matter is made up of black holes, 274 00:17:46,237 --> 00:17:50,673 they would have to be an entirely new type. 275 00:17:50,675 --> 00:17:55,678 It's possible that these black holes are of a type that we've never seen before. 276 00:17:55,746 --> 00:17:59,148 They could be primordial black holes. 277 00:18:00,585 --> 00:18:03,686 [Hopkins] Primordial black holes are an idea. 278 00:18:03,688 --> 00:18:05,488 A theoretical concept at this point 279 00:18:05,490 --> 00:18:10,159 that we've never seen, but they could exist. 280 00:18:10,194 --> 00:18:15,765 If primordial black holes are real then the universe is flooded with black holes. 281 00:18:15,767 --> 00:18:18,367 [Hopkins] The smallest coul have the mass of Mount Evere 282 00:18:18,402 --> 00:18:20,436 packed into the size of one atom. 283 00:18:20,438 --> 00:18:27,443 The biggest could be hundreds of thousands or millions of times the mass of the sun. 284 00:18:27,512 --> 00:18:30,780 [narrator] Stephen Hawking first suggested that primordial black holes 285 00:18:30,782 --> 00:18:35,718 could be dark matter back in the 1970s. 286 00:18:35,720 --> 00:18:40,389 The idea centers on what happened during that intangible momen 287 00:18:40,391 --> 00:18:45,895 13.8 billion years ago, the big-bang. 288 00:18:45,963 --> 00:18:50,466 Theory says that primordial black holes formed in the first fraction 289 00:18:50,468 --> 00:18:52,401 of a second of the early universe. 290 00:18:52,403 --> 00:18:55,104 It's that time between when the universe 291 00:18:55,106 --> 00:19:00,576 goes from a pinprick to this giant inflating ball of gas. 292 00:19:00,578 --> 00:19:03,746 [narrator] In these first moments of the universe's existence, 293 00:19:03,748 --> 00:19:07,150 matter is packed incredibly tightly. 294 00:19:07,185 --> 00:19:10,520 But it's not quite evenly spread. 295 00:19:10,522 --> 00:19:16,225 Even the tiniest fluctuation in density could trigger gravitational collapses. 296 00:19:17,495 --> 00:19:21,330 In other words, black holes would be forming everywhere, 297 00:19:21,866 --> 00:19:24,967 theoretically, in huge numbers. 298 00:19:25,737 --> 00:19:30,506 By the time one second has passed in our universe, 299 00:19:30,508 --> 00:19:33,276 you're already making black holes 300 00:19:33,344 --> 00:19:37,947 thousands, hundreds of thousands of times more massive than our sun. 301 00:19:37,949 --> 00:19:41,517 [narrator] The collective ma of these objects could be va 302 00:19:41,586 --> 00:19:46,622 but could they be 85% of the universe's matter? 303 00:19:46,624 --> 00:19:48,591 [Sutter] If primordial black holes really do exist 304 00:19:48,593 --> 00:19:53,663 there might be enough to explain the dark matter. 305 00:19:53,731 --> 00:20:00,770 [narrator] It's a tantalizin possibility, but there's on pretty big problem. 306 00:20:00,772 --> 00:20:03,739 For most scientists, the physics of the very early universe 307 00:20:03,741 --> 00:20:06,876 is incomplete and hard to trust. 308 00:20:06,878 --> 00:20:11,681 Generations of physicists dismissed primordial black holes 309 00:20:11,749 --> 00:20:17,019 as myths, fantasies, astrophysical unicorns, 310 00:20:17,021 --> 00:20:21,224 until that is, an earthshaki crash in space. 311 00:20:21,793 --> 00:20:24,427 May, 2019. 312 00:20:24,462 --> 00:20:27,463 A violent cosmic event rocks the USA. 313 00:20:27,465 --> 00:20:29,198 How violent? 314 00:20:29,200 --> 00:20:33,236 Well, the physical distance between Louisiana and Washington state 315 00:20:33,271 --> 00:20:39,575 is stretched by nearly the width of an atom which is bigger than it sounds. 316 00:20:39,577 --> 00:20:42,578 The Laser Interferometer Gravitational-Wave Observato 317 00:20:42,580 --> 00:20:45,982 detects this wobble in space time. 318 00:20:46,484 --> 00:20:51,587 This is the biggest gravitational wave event that LIGO has seen. 319 00:20:51,589 --> 00:20:56,559 [narrator] This cosmic disturbance seems to come from colliding black holes 320 00:20:56,561 --> 00:21:02,198 but crucially not the ordina dead star type. 321 00:21:02,233 --> 00:21:06,836 In this LIGO detection, one of the black holes is 85 solar masses. 322 00:21:06,838 --> 00:21:12,375 There's no way that a star could've made that black hole. 323 00:21:12,377 --> 00:21:15,411 [narrator] Physicists believ there's a range of masses 324 00:21:15,413 --> 00:21:20,883 where dying stars can't collapse into black holes. 325 00:21:20,885 --> 00:21:26,822 Instead, stars in this zone become insanely hot and rip themselves apart 326 00:21:26,824 --> 00:21:31,294 leaving nothing to crunch do into a black hole. 327 00:21:32,664 --> 00:21:35,731 Eighty-five solar masses sits right in the middle 328 00:21:35,800 --> 00:21:39,902 of this so called forbidden mass range. 329 00:21:39,904 --> 00:21:43,472 The black hole that LIGO detected can't be a dead star, 330 00:21:43,474 --> 00:21:47,643 but in theory it could be primordial. 331 00:21:47,645 --> 00:21:52,481 Could this discarded theory of dark matter be back in fashion? 332 00:21:52,483 --> 00:21:55,217 The LIGO detections come up and everyone says, 333 00:21:55,286 --> 00:21:57,753 "Oh, right, primordial black holes. 334 00:21:57,755 --> 00:22:00,623 Maybe we should pay more attention to that." 335 00:22:00,691 --> 00:22:03,059 Primordial black holes can be really appealing 336 00:22:03,061 --> 00:22:05,961 because they would solve the dark matter problem. 337 00:22:06,531 --> 00:22:09,699 But unfortunately, it's not that simple. 338 00:22:09,767 --> 00:22:13,803 The thing with flooding the universe with primordial black holes 339 00:22:13,805 --> 00:22:16,339 is that you expect a lot of collisions. 340 00:22:16,341 --> 00:22:18,641 And so LIGO shouldn't have seen one, 341 00:22:18,709 --> 00:22:23,045 it should have seen a thousand of these collisions and we don't. 342 00:22:24,115 --> 00:22:29,318 [narrator] Many scientists doubt what LIGO saw was a primordial black hole 343 00:22:30,288 --> 00:22:34,457 To them, these beasts remai fairytales of physics, 344 00:22:34,459 --> 00:22:38,494 red herrings in the quest for solid evidence of dark matter. 345 00:22:41,833 --> 00:22:43,866 Does dark matter exist? 346 00:22:43,868 --> 00:22:46,635 Or are we chasing shadows? 347 00:22:46,637 --> 00:22:49,472 Some scientists think it's not only real, 348 00:22:49,474 --> 00:22:52,775 but the dark matter is within our grasp, 349 00:22:52,777 --> 00:22:57,279 and that it's flying through our bodies right now 350 00:23:07,125 --> 00:23:11,394 [narrator] We think 85% of the universe's matter is dark. 351 00:23:12,563 --> 00:23:15,698 And yet, we've never found a speck of it. 352 00:23:15,700 --> 00:23:19,802 We can't prove dark matter exists. 353 00:23:19,837 --> 00:23:25,040 Regular matter is made up of everyday particles, like electrons and protons. 354 00:23:26,477 --> 00:23:31,414 Scientists wonder if dark matter is also a type of particle. 355 00:23:32,517 --> 00:23:34,717 One of the leading candidates for dark matter 356 00:23:34,719 --> 00:23:38,454 are these things called Weakly Interacting Massive Particles. 357 00:23:38,522 --> 00:23:41,657 They're massive particles like protons and electrons and things like that. 358 00:23:41,725 --> 00:23:44,927 But they don't interact well with normal matters, so they're weakly interacting. 359 00:23:44,929 --> 00:23:48,097 And they just have this name because it's awesome to call them WIMPs. 360 00:23:49,600 --> 00:23:51,967 [narrator] For decades, scientists have struggled 361 00:23:51,969 --> 00:23:55,971 to find these shy theoretical particles. 362 00:23:55,973 --> 00:23:59,074 [Oluseyi] The very first physics research I ever did in my life 363 00:23:59,076 --> 00:24:04,480 was about actually measuring directly dark matter particles, 364 00:24:04,482 --> 00:24:06,148 these so called WIMPs. 365 00:24:06,150 --> 00:24:10,686 And if they exist, then there will be a flux of millions of them 366 00:24:10,688 --> 00:24:13,856 through my hand right now, just by holding out right here. 367 00:24:13,858 --> 00:24:17,460 If dark matter is actually made of WIMPs, if these particles exist, 368 00:24:17,462 --> 00:24:21,530 then we're actually living basically in a sea of them. 369 00:24:21,532 --> 00:24:25,501 It surrounds and penetrates us and it bin the galaxy together. 370 00:24:26,737 --> 00:24:30,773 [narrator] WIMPs don't play by our rules. 371 00:24:30,775 --> 00:24:33,809 They barely interact with the world of regular matter, 372 00:24:33,811 --> 00:24:36,011 so they're hard to detect. 373 00:24:36,514 --> 00:24:38,781 But when they play with each other, 374 00:24:38,849 --> 00:24:45,221 sparks fly, intense flashes that we just might be able to see. 375 00:24:45,957 --> 00:24:49,892 [Plait] As the theory goes, WIMPs will self-annihilate. 376 00:24:49,894 --> 00:24:53,028 WIMP A and WIMP B get too close together, poof, 377 00:24:53,030 --> 00:24:55,331 they explode and they create gamma rays. 378 00:24:56,367 --> 00:25:01,103 [narrator] Gamma rays are high energy light, making them easy to spot. 379 00:25:03,541 --> 00:25:07,710 Scientists point their detectors at the cente of the Milky Way, 380 00:25:07,712 --> 00:25:12,948 where they believe the WIMP collision rate should be especially high. 381 00:25:13,017 --> 00:25:15,217 [Plait] We have a 4 million solar mass black hole there 382 00:25:15,219 --> 00:25:16,485 There are billions of stars there. 383 00:25:16,553 --> 00:25:20,756 That's where most of the mass of the galaxies is densest. 384 00:25:20,824 --> 00:25:24,460 So any WIMPs orbiting the galaxy will feel this natural attraction 385 00:25:24,462 --> 00:25:26,896 towards the center and fall toward it. 386 00:25:26,898 --> 00:25:29,798 [narrator] The Fermi Large Area Telescope scoured 387 00:25:29,867 --> 00:25:33,269 the center of our galaxy for more than 10 years. 388 00:25:33,671 --> 00:25:36,338 It detected lots of gamma rays, 389 00:25:36,340 --> 00:25:41,410 but scientists couldn't tel if they came from colliding WIMPs. 390 00:25:41,412 --> 00:25:45,014 The Galactic Center is a mess. It's like downtown of a city, right? 391 00:25:45,016 --> 00:25:47,483 That's where everything is, where all the hustle and bustle is. 392 00:25:47,485 --> 00:25:49,451 There are stars exploding there, 393 00:25:49,520 --> 00:25:51,887 just tons of stars, gas, magnetic fields, a black hole, 394 00:25:51,889 --> 00:25:53,656 a lot of sources of gamma rays, 395 00:25:53,658 --> 00:25:56,458 so it's very difficult to tease out the signal. 396 00:25:56,460 --> 00:25:59,061 [narrator] Downtown Milky Wa was a washout. 397 00:25:59,063 --> 00:26:01,730 So the scientists turned their attention to planets 398 00:26:01,799 --> 00:26:03,866 living in less noisy ZIP codes, 399 00:26:03,868 --> 00:26:07,970 where WIMP collisions should be easier to spot. 400 00:26:07,972 --> 00:26:10,639 One place where you might see evidence for WIMP collisions 401 00:26:10,641 --> 00:26:12,908 is actually the cores of exoplanets. 402 00:26:12,910 --> 00:26:19,148 Turns out exoplanets might be the best dark matter detector we have. 403 00:26:20,017 --> 00:26:22,885 You can use giant planets orbiting distant stars 404 00:26:22,887 --> 00:26:26,155 as laboratories to understand dark matter. 405 00:26:26,891 --> 00:26:30,526 [narrator] We know gravity should attract WIMPs. 406 00:26:30,594 --> 00:26:35,764 The more gravity, the more dark matter particl come together. 407 00:26:35,766 --> 00:26:39,969 Scientists suggest that WIMP congregate inside the cores 408 00:26:40,037 --> 00:26:42,805 of the Milky Way's largest gas planets. 409 00:26:42,807 --> 00:26:45,674 In these super-sized gas giants, 410 00:26:45,676 --> 00:26:51,413 WIMPs could collide, annihilate, and release gamma rays. 411 00:26:51,415 --> 00:26:53,482 If there are these WIMPs that are collecting the centers 412 00:26:53,484 --> 00:26:56,018 of mass of exoplanets, the annihilation of that dark matter 413 00:26:56,020 --> 00:26:57,886 can heat those exoplanets up. 414 00:26:57,888 --> 00:27:02,291 If you have a WIMP-heated exoplanet, and that's just fun to say, 415 00:27:02,359 --> 00:27:03,859 this thing is going to be warm, 416 00:27:03,861 --> 00:27:06,829 it's gonna be warmer than the heat of space, which is very cold. 417 00:27:06,831 --> 00:27:09,164 So what you need is an infrared telescope, 418 00:27:09,166 --> 00:27:11,700 something that sees an infrared light and is sensitive enough 419 00:27:11,702 --> 00:27:14,803 to be able to measure the temperatures of these things. 420 00:27:14,872 --> 00:27:20,042 [narrator] But a dedicated telescope like this won't launch until 2028. 421 00:27:21,746 --> 00:27:25,547 For some dark matter hunters that's too long to wait. 422 00:27:25,549 --> 00:27:29,485 They argue that WIMPs do have one characteristic 423 00:27:29,487 --> 00:27:33,155 that should allow us to detect them right here on Earth. 424 00:27:33,891 --> 00:27:37,826 The key to detecting WIMPs is in their name, it's the W-I. 425 00:27:37,828 --> 00:27:40,829 They're weakly interacting. They're not not interacting. 426 00:27:40,831 --> 00:27:43,599 They do interact, it's just very weak with matter. 427 00:27:43,634 --> 00:27:46,502 And that means that there are the rare occasions 428 00:27:46,504 --> 00:27:48,771 where it will smack into a particle of normal matter 429 00:27:48,839 --> 00:27:51,273 and then there are effects that we can observe. 430 00:27:52,777 --> 00:27:58,414 [narrator] Scientists in Gran Sasso in Central Italy watch for a spark of energy 431 00:27:58,449 --> 00:28:02,685 generated when a WIMP hits an atom of regular matter. 432 00:28:02,720 --> 00:28:06,522 Their detector, a tank of super cooled xenon 433 00:28:06,524 --> 00:28:11,493 built thousands of feet beneath the Earth's surface 434 00:28:11,495 --> 00:28:14,997 The beauty of putting this detector under a mountain is that you've got all of this 435 00:28:14,999 --> 00:28:16,732 rock and soil and everything else 436 00:28:16,734 --> 00:28:18,801 which is blocking a lot of background noise. 437 00:28:18,836 --> 00:28:20,936 When you're looking for a WIMP interaction, 438 00:28:20,938 --> 00:28:24,573 you're looking for something that's very rare and something very subtle, 439 00:28:24,575 --> 00:28:27,009 so you don't want other things going on. 440 00:28:27,044 --> 00:28:29,411 You don't want other particles coming in and messing up your experiment. 441 00:28:29,480 --> 00:28:33,849 These Weakly Interacting Massive Particles will pass right through that mountain, 442 00:28:33,917 --> 00:28:36,418 and then if they smack into a xenon atom, we can look at it and go, 443 00:28:36,420 --> 00:28:39,388 "Ah, that was a dark matter particle." 444 00:28:39,390 --> 00:28:44,927 [narrator] Detecting a WIMP could be definitive proof that dark matter exists. 445 00:28:44,995 --> 00:28:49,898 In 2020, the scientists spotted something in the results. 446 00:28:49,900 --> 00:28:52,301 But was it the elusive evidence 447 00:28:53,003 --> 00:28:56,205 or a ghost among the stars? 448 00:29:03,614 --> 00:29:06,749 [ambient music playing] 449 00:29:06,751 --> 00:29:09,284 [narrator] Scientists believ they can prove dark matter is real 450 00:29:09,320 --> 00:29:12,621 by detecting WIMPs. 451 00:29:12,623 --> 00:29:16,792 An experiment buried deep beneath an Italian mountain 452 00:29:16,794 --> 00:29:21,096 spotted unusual activity in a tank of regular matter 453 00:29:21,098 --> 00:29:22,898 pure liquid xenon. 454 00:29:22,900 --> 00:29:25,000 [popping] 455 00:29:26,470 --> 00:29:29,972 So a WIMP detector, like the XENON1T, 456 00:29:30,007 --> 00:29:33,909 waits for a little WIMP, tiny, tiny little particle 457 00:29:33,977 --> 00:29:36,745 to hit an atom of normal matter, 458 00:29:36,747 --> 00:29:38,747 and that creates a vibration. 459 00:29:38,749 --> 00:29:42,718 And we can see this entire block of xenon shake 460 00:29:42,786 --> 00:29:46,255 just a little bit from that little, subatomic collision. 461 00:29:47,458 --> 00:29:49,191 [narrator] The intensity of the vibration 462 00:29:49,193 --> 00:29:52,127 from the particle collision is critical. 463 00:29:52,129 --> 00:29:58,534 In theory, a WIMP striking a xenon atom should generat a powerful shock. 464 00:29:58,536 --> 00:30:03,405 The vibrations XENON1T detected were too weak. 465 00:30:05,042 --> 00:30:08,010 When a WIMP comes through, it smashes into the atom. 466 00:30:08,012 --> 00:30:11,513 It seemed like here something was just sort of rattling the electrons 467 00:30:11,515 --> 00:30:13,248 on the outside of the atom. 468 00:30:13,283 --> 00:30:15,584 So whatever is causing these detections was likely 469 00:30:15,586 --> 00:30:18,487 something much smaller than a WIMP. 470 00:30:18,489 --> 00:30:21,290 [Sutter] Let's take these results at face value 471 00:30:21,358 --> 00:30:23,926 It... If they're correct, it's telling us 472 00:30:23,994 --> 00:30:26,428 that the dark matter isn't a WIMP, 473 00:30:26,463 --> 00:30:28,397 but something much, much smaller 474 00:30:28,399 --> 00:30:30,332 and something much, much lighter. 475 00:30:31,368 --> 00:30:34,469 [narrator] The results sugge that what hit the xenon 476 00:30:34,471 --> 00:30:40,776 was actually a much smaller theoretical particl called an axion. 477 00:30:40,844 --> 00:30:44,146 [Bullock] Axions are really weird particles, incredibly light. 478 00:30:44,148 --> 00:30:47,916 In fact, almost zero mass is possible for an axion. 479 00:30:47,918 --> 00:30:50,519 An axion is no bigger 480 00:30:50,587 --> 00:30:55,457 than 150 billionth the size of an electron. 481 00:30:55,459 --> 00:31:02,731 Compared to a WIMP, an axion is like a soccer ball compared to our sun. 482 00:31:02,733 --> 00:31:05,534 [narrator] The sheer tinines of axions makes them seem 483 00:31:05,536 --> 00:31:08,070 like an unlikely candidate for dark matter. 484 00:31:09,740 --> 00:31:14,977 If dark matter is real, it makes up 85% of the matte in the universe. 485 00:31:18,015 --> 00:31:24,152 To account for all that mass we would need an almost unfathomable number of axion 486 00:31:24,955 --> 00:31:28,590 142 trigintillion of them, in fact. 487 00:31:28,592 --> 00:31:32,027 That's 140 with 93 zeros after it. 488 00:31:33,364 --> 00:31:37,833 If axions exist, space must swimming with them. 489 00:31:37,835 --> 00:31:42,437 They must be packed into every corner of the cosmos. 490 00:31:42,506 --> 00:31:46,008 When regular matter clumps together, it forms stars. 491 00:31:46,644 --> 00:31:49,611 So, to prove dark matter exists, 492 00:31:49,613 --> 00:31:52,447 maybe we should be looking for dark stars. 493 00:31:53,984 --> 00:31:56,218 There's no reason they can't exist. 494 00:31:56,220 --> 00:31:58,053 There's even a name for them 495 00:31:58,923 --> 00:32:00,389 Ghost stars. 496 00:32:01,558 --> 00:32:03,825 [Bullock] They're very weird objects. 497 00:32:03,827 --> 00:32:07,930 These ghost stars are like nothing we would ever see in the night sky. 498 00:32:07,932 --> 00:32:09,431 [narrator] We've never seen a ghost star. 499 00:32:09,433 --> 00:32:14,870 They are theoretical object made of hypothetical axions 500 00:32:14,872 --> 00:32:20,208 But in theory, ghost stars should form like any other star, 501 00:32:20,243 --> 00:32:22,544 pulled together by gravity. 502 00:32:22,546 --> 00:32:28,417 They would be gigantic, super dense objects floating through space. 503 00:32:28,419 --> 00:32:32,321 They could reach the mass of tens of millions of suns 504 00:32:32,823 --> 00:32:35,891 But because they are made of dark matter, 505 00:32:35,926 --> 00:32:40,696 ghost stars would produce no energy and emit no light 506 00:32:40,698 --> 00:32:45,667 They would be transparent to both light and matter. 507 00:32:45,669 --> 00:32:48,870 If you were right next to it, you wouldn't even notice it, right? 508 00:32:48,905 --> 00:32:52,474 If we sent a probe through it, it'd sail right through it 509 00:32:52,476 --> 00:32:55,911 uh, and once it passed through, it would be pulled back by its gravity. 510 00:32:55,946 --> 00:33:02,584 [narrator] 85% of the matte in our universe could consis of transparent orbs 511 00:33:02,586 --> 00:33:07,322 made of infinitesimally small, dark matter particles 512 00:33:08,058 --> 00:33:11,226 But do these invisible stars exist? 513 00:33:11,294 --> 00:33:13,462 The evidence is thin, but.. 514 00:33:13,464 --> 00:33:14,763 [tape rewinding] 515 00:33:14,765 --> 00:33:18,433 [narrator] Rewind back to th LIGO detection in 2019. 516 00:33:21,905 --> 00:33:25,173 The gravitational wave detector picked up the signa 517 00:33:25,175 --> 00:33:28,377 of two massive objects colliding. 518 00:33:28,946 --> 00:33:30,379 [pulsating explosion] 519 00:33:31,382 --> 00:33:34,983 We call the event GW190521. 520 00:33:34,985 --> 00:33:38,854 Most scientists agree this w a black hole collision. 521 00:33:38,856 --> 00:33:43,058 But could it have been clashing ghost stars? 522 00:33:43,794 --> 00:33:45,994 [Plait] If there are ghost stars out there 523 00:33:45,996 --> 00:33:47,729 and they can interact with each other gravitationally, 524 00:33:47,731 --> 00:33:49,898 they may collide. 525 00:33:49,900 --> 00:33:51,833 And when they do, they would emit gravitational waves 526 00:33:51,869 --> 00:33:54,936 and it would look a lot like two black holes colliding. 527 00:33:54,938 --> 00:34:00,342 In fact, it would look theoretically very much like GW190521. 528 00:34:01,678 --> 00:34:04,346 [narrator] One collision, two explanations. 529 00:34:05,916 --> 00:34:09,384 Primordial black holes or ghost stars, 530 00:34:10,821 --> 00:34:13,488 LIGO can't tell them apart. 531 00:34:13,490 --> 00:34:18,994 Do these ideas bring us closer to proving the existence of dark matter 532 00:34:18,996 --> 00:34:25,133 Or are we just hurtling further down a weird physics rabbit hole 533 00:34:25,135 --> 00:34:29,571 [Chiara Mingarelli] Primordial black holes, ghost stars, axions, 534 00:34:29,639 --> 00:34:32,140 this is all very exotic physics. 535 00:34:32,176 --> 00:34:36,745 We can't take for granted that any of this is real or that it's not real. 536 00:34:36,747 --> 00:34:37,612 We just don't know. 537 00:34:37,614 --> 00:34:40,549 Dark matter is irritating. 538 00:34:40,551 --> 00:34:43,752 [groans] We know it's out there. We see its effects, right? 539 00:34:43,787 --> 00:34:48,924 But we can't see the dark matter and that's frustrating. 540 00:34:48,992 --> 00:34:51,893 And it's like a lot of young fields in astronomy. 541 00:34:51,929 --> 00:34:56,264 We have way more ideas than we do hard observations. 542 00:34:57,468 --> 00:35:00,402 [narrator] We have ideas, we have theories, 543 00:35:00,404 --> 00:35:05,440 but without direct observations, we just can't back them up with solid proo 544 00:35:06,844 --> 00:35:11,313 The more we look, the harder it is to find dark matter. 545 00:35:12,549 --> 00:35:16,818 Maybe it's primordial black holes from the early universe. 546 00:35:16,820 --> 00:35:22,290 Maybe it's a sea of particle that flow right through us every day. 547 00:35:22,626 --> 00:35:28,630 Or maybe it's gigantic, transparent ghost stars. 548 00:35:28,632 --> 00:35:34,202 Perhaps it's the combined ma of Santa's sleigh and the Easter Bunny's baske 549 00:35:34,204 --> 00:35:39,441 Or maybe all our physics is based on questionable mat 550 00:35:49,553 --> 00:35:54,856 [narrator] 85% of the stuff in the universe is missing in action. 551 00:35:54,858 --> 00:35:58,226 The search for this dark matter looks hopeless. 552 00:35:59,696 --> 00:36:03,064 This problem of dark matter is really a tough one. 553 00:36:03,066 --> 00:36:04,165 Everything that we've predicted 554 00:36:04,167 --> 00:36:07,169 and then gone and looked for, we're not finding. 555 00:36:07,171 --> 00:36:09,704 It's starting to become a huge embarrassment. 556 00:36:09,706 --> 00:36:11,907 Surely something so fundamental 557 00:36:11,909 --> 00:36:13,808 to our cosmology should be detectable. 558 00:36:13,810 --> 00:36:15,577 And yet, it remains elusive. 559 00:36:17,981 --> 00:36:22,584 [narrator] We're stumbling blindly around the limits of our understanding. 560 00:36:22,586 --> 00:36:26,288 As of right now, there are zero direct observations. 561 00:36:27,624 --> 00:36:31,059 Maybe dark matter doesn't exist after all. 562 00:36:31,061 --> 00:36:33,895 Instead of searching for an invisible substance 563 00:36:33,897 --> 00:36:36,765 affecting the universe with its gravity, 564 00:36:36,767 --> 00:36:41,736 maybe it's gravity we don't quite understand. 565 00:36:41,738 --> 00:36:45,941 If you're looking in a galaxy and it's spinning way too quickly, 566 00:36:45,943 --> 00:36:50,045 either there's a new ingredient in the galaxy, 567 00:36:50,047 --> 00:36:52,581 like dark matter, that holds it all together. 568 00:36:52,583 --> 00:36:56,818 Or you're misunderstanding the laws of physics. 569 00:36:56,820 --> 00:36:58,720 [narrator] To describe the effects of gravity, 570 00:36:58,722 --> 00:37:04,326 we use the nearly 350-year-o math of Sir Isaac Newton. 571 00:37:05,929 --> 00:37:10,465 Maybe to explain the excess gravity we see in the universe, 572 00:37:10,467 --> 00:37:13,168 it's not extra matter we nee 573 00:37:13,470 --> 00:37:15,503 It's better math. 574 00:37:15,505 --> 00:37:17,606 Although we understand very well how gravity works 575 00:37:17,608 --> 00:37:19,874 here on Earth and in our Solar System, 576 00:37:19,876 --> 00:37:22,744 perhaps when you get up to galactic scales, 577 00:37:22,746 --> 00:37:25,580 it actually behaves just slightly differently. 578 00:37:25,582 --> 00:37:29,084 And if that were the case, you can kind of tweak that idea 579 00:37:29,086 --> 00:37:33,154 until it fits the data we see of how galaxies are spinning around 580 00:37:33,156 --> 00:37:36,024 without needing dark matter. 581 00:37:36,927 --> 00:37:38,393 [narrator] Questioning the math of a legend 582 00:37:38,395 --> 00:37:41,363 of physics might sound like sacrilege, 583 00:37:41,365 --> 00:37:45,600 but to solve the dark matter conundrum, it has been done. 584 00:37:45,602 --> 00:37:50,805 It's called Modified Newtonian Dynamics or MOND. 585 00:37:50,807 --> 00:37:56,311 Modeling galaxies with this math produces very different results. 586 00:37:57,748 --> 00:38:01,850 On its surface, MOND is not a bad idea. 587 00:38:01,852 --> 00:38:04,753 In the same way that we would normally program a computer 588 00:38:04,755 --> 00:38:07,589 to include dark matter in our simulations, 589 00:38:07,591 --> 00:38:10,492 you can take that out, and instead program it 590 00:38:10,494 --> 00:38:12,927 with a different law of gravity with MOND. 591 00:38:12,929 --> 00:38:16,831 And then, you can set up a kind of spinning mass of gas 592 00:38:16,833 --> 00:38:19,567 and it does seem to be possible with MOND 593 00:38:19,569 --> 00:38:23,171 to get things settled down and look a bit like a real galaxy. 594 00:38:25,976 --> 00:38:29,477 [narrator] Changing the law of gravity accurately recreates 595 00:38:29,479 --> 00:38:34,015 the super-fast spin astronomers see through their telescopes. 596 00:38:34,017 --> 00:38:37,485 No need for dark matter. It doesn't exist. 597 00:38:37,921 --> 00:38:39,154 Case closed? 598 00:38:39,589 --> 00:38:40,889 Not by a long shot. 599 00:38:40,891 --> 00:38:43,458 With anything bigger than a galaxy, 600 00:38:43,460 --> 00:38:46,995 this artificial physics breaks down. 601 00:38:47,798 --> 00:38:50,999 MOND does really well on galaxy scales, 602 00:38:51,001 --> 00:38:52,801 but when you zoom out and you go to larger 603 00:38:52,803 --> 00:38:55,103 and larger structures in our universe, 604 00:38:55,105 --> 00:38:59,174 like clusters of galaxies, big, big structure, 605 00:38:59,176 --> 00:39:03,278 you see that MOND by itself can't reproduce all of our observations. 606 00:39:03,280 --> 00:39:04,779 There's something missing. 607 00:39:04,781 --> 00:39:05,680 Dark matter. 608 00:39:05,949 --> 00:39:07,382 Dark matter, dark matter, dark matter. 609 00:39:07,384 --> 00:39:09,017 Dark matter. 610 00:39:09,019 --> 00:39:13,355 In MOND, you still have to invoke the existence of material you can't see. 611 00:39:13,990 --> 00:39:15,657 [Andrew Pontzen] It basically introduces 612 00:39:15,659 --> 00:39:17,859 some of its own dark matter as well, 613 00:39:17,861 --> 00:39:21,363 which kind of negates the point of having MOND in the first place. 614 00:39:24,434 --> 00:39:28,069 [narrator] MOND doesn't replace dark matter. 615 00:39:28,071 --> 00:39:31,940 The universe still needs something to hold it togethe 616 00:39:31,942 --> 00:39:33,842 We just don't know what it i 617 00:39:33,844 --> 00:39:37,245 But there are plenty of new ideas flying around. 618 00:39:38,515 --> 00:39:41,416 In my theory, the dark matter is a super fluid. 619 00:39:42,853 --> 00:39:45,920 [narrator] It's a radical new theory of dark matter, 620 00:39:45,989 --> 00:39:48,790 particles not acting individually, 621 00:39:48,792 --> 00:39:54,028 but flowing as one invisibl mass around the galaxies. 622 00:39:54,731 --> 00:39:58,633 A super fluid is like an ordinary fluid that flows, 623 00:39:58,635 --> 00:40:02,137 but in this case, it flows without any resistance or viscosity. 624 00:40:02,139 --> 00:40:05,440 If I pour honey, it will flow very slowly. 625 00:40:05,442 --> 00:40:06,741 It has high viscosity. 626 00:40:06,743 --> 00:40:10,745 A super fluid will just flo and never stop flowing. 627 00:40:10,747 --> 00:40:14,849 [narrator] As the super flui dark matter flows around the universe, 628 00:40:14,851 --> 00:40:21,122 eddies and waves form large enough to engulf entire galaxies. 629 00:40:21,124 --> 00:40:24,959 The gravity of the fluid holds the stars together. 630 00:40:25,996 --> 00:40:31,699 But like most theories on dark matter, there's no direct evidence. 631 00:40:31,701 --> 00:40:35,136 If these waves are on the size of galaxies, 632 00:40:35,138 --> 00:40:39,774 then we have to find detectors that can detect those types of huge waves. 633 00:40:39,776 --> 00:40:41,976 They don't exist at the moment. 634 00:40:42,913 --> 00:40:45,680 [narrator] Which brings us back to square one. 635 00:40:45,682 --> 00:40:48,616 We just can't prove that dark matter is real. 636 00:40:48,618 --> 00:40:51,719 Primordial black holes, ghost stars, WIMPs, 637 00:40:51,721 --> 00:40:55,990 a super fluid sloshing about the cosmos, 638 00:40:55,992 --> 00:40:59,060 or maybe we're just using the wrong math. 639 00:41:00,630 --> 00:41:02,864 What's your money on? 640 00:41:02,866 --> 00:41:07,368 If I had to wager $20 on what dark matter is. 641 00:41:07,704 --> 00:41:08,870 Hmm. 642 00:41:08,939 --> 00:41:12,707 I would never place money on what dark matter is. 643 00:41:12,709 --> 00:41:14,442 I just think we have no idea. 644 00:41:15,178 --> 00:41:20,114 My money is on dark matter itself is real, 645 00:41:20,116 --> 00:41:22,217 but it's not the whole picture. 646 00:41:22,219 --> 00:41:26,254 I would say left socks in dryers. 647 00:41:26,256 --> 00:41:30,725 I would say remote controls that fall into sofa cushions and disappear. 648 00:41:30,727 --> 00:41:32,093 I would love there to be 649 00:41:32,095 --> 00:41:35,797 dark matter, ghost stars, planets, even dark matter people. 650 00:41:35,799 --> 00:41:37,465 I'm going all black. 651 00:41:37,968 --> 00:41:43,037 I think no current ideas are correct. 652 00:41:43,039 --> 00:41:47,342 I think dark matter is something that we haven't thought of yet. 653 00:41:48,979 --> 00:41:50,512 [narrator] Does dark matter exist? 654 00:41:51,314 --> 00:41:52,947 Watch the space.