All language subtitles for How the Universe Works s07e07 Battle of the Dark Universe.eng

narrator: Across the universe,
an endless war rages,

a bitter struggle
between invisible forces.

Tegmark: We've seen this
cosmic battle go on

for the past 14 billion years.

Like two navies, fighting it out
in the ocean of space time.

[ crackling ]

narrator:
Dark matter...

And dark energy

battling for control
of the universe.

They've shaped the entire
history of the universe.

They're not about to stop now.

Narrator: Shadowy forces
dictating our past,

our present, and our future.

Thaller: The dark universe
is kind of a puppet master

behind the scenes,

guiding the universe
that we can see.

Oluseyi: It's taken over
the universe and, eventually,

it may well
destroy the universe.

[ rumbling ]

[ crackling, bursting ]

-- captions by vitac --
www.Vitac.Com

captions paid for by
discovery communications

narrator: As we look out
across the universe,

we see nebulas,

stars,

and planets,

all constructed
from visible matter.

But what we see is just
a small part of the cosmos.

The rest is invisible,

unknown...

The dark universe.

Dark universe is a very common
phrase to describe our universe,

because it turns out
most of our cosmos is dark.

Dark not as in night.

Dark as in,
doesn't interact with light,

and dark as in, we don't
really understand it.

All the objects
in our daily experience --

our bodies, the air, the chair
that you're sitting in,

the planets and stars,

all of that only adds up
to 5 percent of the universe.

It's the other 95 percent
that is the dark stuff,

the dark universe.

narrator: The dark universe
is made of two forces --

dark matter and dark energy.

In a way, the dark matter
and dark energy

sort of oppose each other.

[ whooshing ]

dark matter has positive gravity
that pulls things together,

and dark energy
has repulsive gravity

that pushes things apart.

Freese: To encapsulate that
in one sentence,

dark matter is attractive,
dark energy is repulsive.

Narrator:
Since the dawn of time,

two forces slugging it out
for control of the universe.

Dark matter and dark energy

are locked
in this epic struggle.

The dark matter is trying
to bring things together.

The dark energy is trying
to drive everything apart.

So it's basically a battle.
Who's going to win?

[ rumbling ]

narrator:
It's a struggle that started

13.8 billion years ago

in the cosmic furnace
of the big bang.

the infant universe was a super
hot ball of intense radiation,

but suddenly...

[ bursting ]
...It started to transform.

It cooled and expanded,

triggering the birth
of the regular universe

and the dark universe.

Scientists believe that both
dark matter and dark energy

formed in the first moments
of the big bang.

It would have formed probably
just fractions of a second

after the big bang,

around the time
that normal matter formed,

and the processes that created

the normal matter
we know all about.

Similar processes probably
created the dark matter.

narrator: During these
first microseconds,

the universe was a hot, dense
ball of matter and energy.

They're two sides
of the same coin.

Matter can convert
into energy...

[ crackling ]

...And energy can convert
directly into matter,

visible particles of regular
matter forming atoms,

planets, stars, and us --

and other particles.

They are invisible.

They are dark matter.

One of the big mysteries that we
as astronomers have to solve

is what this dark matter is.

We just don't know.

The idea of a type of matter
that you can't see

and that acts differently
than normal matter

is sort of out there.

It's sort of weird.

Narrator: But the dark universe
does leave clues.

It's like a crime scene.

You know that a crime
has been committed,

even though you don't know
the perpetrator yet.

We see the hints.
We see the signs.

We see the signals
that something funny

is happening
in our universe,

even though we don't know
exactly what's causing it.

Narrator:
There are several contenders

for what dark matter could be.

It could be normal matter
that we just don't see,

or it could be some
sort of exotic matter,

a particle of some sort
that we haven't detected yet.

Sutter: It could be
a species of particle,

like an electron,
like a proton,

like a quark,

but a special kind that
doesn't interact with light.

One of the favorite possible
models of dark matter

are wimps, weakly interacting
massive particles.

narrator: Wimps may not
interact strongly

with other matter or light,

but they do exert
a gravitational pull.

[ crackling ]

so they're the best candidates
for the particles of dark matter

that formed in the big bang.

the early universe was
intensely hot and dense,

full of new particles
of both visible matter

and invisible dark matter.

But another force
was present --

dark energy.

Dark energy has existed
since the time of the big bang.

Freese:
Dark energy was always there.

We don't know how much
of it there was,

but it's possible that
the same amount of dark energy

was always there.

[ rumbling ]

narrator: Our understanding of
what that dark energy is

is very limited.

Sutter:
If you were to ask a roomful of 10 theoretical physicists

on the nature of dark energy,

you'd get about
12 different answers.

We're not sure
what dark energy is.

Dark energy is just a fancy
name for our ignorance.

Dark energy is nothing
more than a placeholder name

for this enormous gap
in our understanding

of how the cosmos works.

But we don't understand
it at all.

[ chuckles ] it's true.

narrator: One potential answer
to what dark energy

actually is may be found
in so-called empty space.

Could this be the source
of dark energy?

Tegmark: We used to think of
space as just boring emptiness.

But now, I think it's healthier
to think of space

as a kind of substance.

Narrator:
A substance that carries

a strange type of energy.

Every small region of space

has a little bit
of energy in it,

just associated
with the vacuum itself.

Vacuum energy is the idea
that vacuum isn't empty,

that there is something there.

There's an energy in it
with a kind of antigravity.

[ crackling ]

narrator:
Perhaps this vacuum energy

that pushes against gravity

is the mysterious dark energy.

We simply don't know.

We're not sure that the dark
energy is the vacuum energy.

It could be a new type of energy
that permeates all of space.

This is what we're
trying to measure now.

Narrator:
Dark energy and dark matter,

forged in the intense heat
of the big bang,

opposing forces,
one attractive, one repulsive.

But together,
over 13.8 billions years,

they will shape
the history of the universe.

[ rumbling ]

[ rumbling ]

narrator: The story of
the universe is dominated

by two powerful
opposing forces --

dark matter and dark energy.

For 13.8 billions years,

they've battled it out
for control of the cosmos.

Dark matter and dark energy
are out there,

and they've shaped the entire
history of the universe.

Our universe is actually
the balance between dark forces.

Dark matter is trying to draw
everything together,

and dark energy is trying
to rip everything apart.

Narrator: After the big bang,
the infant universe was small,

intensely hot
and intensely dense.

[ whooshing, rumbling ]

dark matter, the force bent
on bringing things together,

thrived.

But in this compressed space,
dark energy,

the force trying to drive things
apart, had no room to act.

When things were
closer together,

the density of matter
and radiation was bigger,

so big that the dark energy
didn't matter.

Narrator: The environment was
also tough for normal matter.

It was so hot,
intense radiation

prevented visible matter from
bunching together to form atoms.

If any normal matter
tried to clump together

through gravity
or some other force,

this energy would just
basically blast it apart.

In the very early universe,

when our universe
was a lot smaller

and a lot hotter
and a lot denser,

matter, normal matter,
tried to collect together,

wanted to join the party.

But it was prevented
from doing so

because there was also radiation
that would throw it out.

Narrator:
Unable to stick together,

normal visible matter
sped out

across the infant cosmos
in a blizzard of particles.

[ crackling ]

but then dark matter,

the force that brings things
together, intervened.

[ crackling ]

dark matter doesn't talk
to radiation,

doesn't talk to light.

Can do whatever it wants.

It starts clumping together.

Narrator: Radiation
pushes normal matter apart,

stopping it
from forming dense regions.

But photons simply pass straight
through the dark matter,

allowing it to clump and fall
into dense pockets or wells.

The dark matter begins to clump
together gravitationally,

and this means that the matter
is going to fall into

those dark matter wells.

Narrator: Over time,
more and more regular matter

is pulled into
the dark matter wells.

The regions that have
a little bit more stuff,

gravity makes them bigger,

and the regions that have
less stuff, those expand more.

So you have little pockets
of slight extra matter,

have more and more
and more matter over time.

[ rumbling ]

narrator: Gravity-rich pockets
of dark matter

pull particles
of regular matter together.

[ whistling, rumbling ]

gradually, they form
giant clouds of hydrogen

and helium gas.

Dark matter has laid the
foundation stones of the cosmos.

This force may be dark,
but it's highly creative.

and now, 180 million years
after the big bang,

everything is in place

for the next dark matter
construction milestone --

the creation of stars.

We know stars,

in the very early universe
at the edge of time,

had to form from
the collapse of gas clouds

under their own gravity.

narrator:
But there's a problem.

The clouds of hydrogen in the
infant universe can't collapse,

and no collapse
means no stars.

Bullock: The gas in the early
universe has a lot of pressure,

and this pressure
keeps it from collapsing.

The dark matter
doesn't experience

that kind of pressure.

So the dark matter can clump up

and make sites
for structure formation.

Narrator: So dark matter comes
to the rescue,

creating regions
of higher gravity,

dragging in hydrogen gas,

forcing the clouds to get
denser and denser,

creating the conditions
for collapse and then creation.

[ rumbling ]

so it's only when the gravity
of the dark matter

overwhelms the pressure of gases

that the gases can collapse
and turn into stars.

Bullock: As soon as the gas
cools down,

it can fall into those
potential wells

that the dark matter created,

almost like little nurseries
for stars,

and they start
forming in earnest.

narrator: Dark matter provides
a boost of gravity

to kick-start hydrogen into
constructing the first stars,

stars that are the seeds
of the first galaxies.

Tremblay: So it's dark matter
that would have coalesced

in the early universe
and grown from there,

and then the luminous
component of the universe,

the things that we think of
as being the universe itself,

like stars and galaxies,

would have just been along
for the ride.

Without the presence of dark
matter to seed structures,

there wasn't enough time in the
early universe to form galaxies,

which means you and me
have to thank dark matter

for our existence.

Narrator:
And dark matter now begins

a much more ambitious
architectural project --

to shape the entire
universe itself,

to build the biggest structure
ever constructed --

the cosmic web.

[ rumbling ]

narrator: The war between
dark matter and dark energy

has been raging
since the birth of time.

[ rumbling ]

but in the early years,
it's a one-sided contest.

In the early universe, the only
thing that really mattered

was the dark matter
and the normal matter.

Narrator: Dark matter,

the force that brings things
together,

is in the driver's seat.

[ rumbling ]

dark energy, the force
that pulls things apart,

is the underdog.

We've seen this
cosmic battle go on

for the past 14 billion years.

Fortunately for us, the dark
energy got off to a slow start.

Narrator: Meanwhile, dark matter
is busy at work,

building the universe.

Not only does it trigger
the birth of the first stars,

it embarks on an even
more formidable

construction project --

the cosmic web.

Straughn:
There's this large-scale structure of filaments

that galaxies
seem to form on,

and that's what we call
the cosmic web.

And we can trace the formation
of this cosmic web

all the way back
to the early universe.

Now this is such a huge
structure,

we don't think there's time
in the universe

for matter's gravity alone
to do this.

There must have been
an underlying scaffold

of dark matter.

Plait: The dark matter started
forming into these filaments,

and when the universe
cooled enough,

normal matter
could start to stream

into this gravitational
attraction of the dark matter.

That became the scaffolding

on which this large-scale
structure was built.

Narrator: The filaments of
dark matter joined together,

drawing in more
and more hydrogen gas.

Dense clouds of gas build up at
the junctions of the filaments,

the point where gravity
is at its strongest.

slowly and surely,

a familiar-looking structure
starts to take shape.

If you've ever gone outside,

and you can see a spider web
covered in dew,

that's kind of like
what happened with the universe.

In this case, the spider web

is the structure
of the dark matter.

It's all of these filaments,
and the moisture in the air

is what condenses around them,

just like the normal matter
fell into the dark matter web

to form these gigantic
structures in the universe.

Narrator: Dark matter,
the universe's master builder,

succeeds in stitching
together a cosmic web.

This will be the framework
for the entire universe.

And so it is dark matter
that would choreograph

and sculpt the shape
of the universe itself.

One of the amazing things
about dark matter is,

without it, we wouldn't be here.

It's hard to imagine how you
could have structure

in the universe
without dark matter.

Narrator: Galaxies,
and then galactic clusters,

form at the junctions
of the filaments.

Slowly, but surely, the universe
begins to take shape.

When we look at this structure
over a cosmic scale,

we see that it looks
kind of like a sponge.

You see voids with galaxies
all over the edges of them.

That is the structure
that was formed

by the dark matter
in the early universe.

Dark matter is the thing
that enabled,

that provided enough gravity

for the initial seeds of
structure formation to coalesce,

for galaxies themselves to form.

[ rumbling ]

and, of course,
without galaxies,

there are no stars,
and there's no planets,

and there's no us.

Narrator:
For 9 billion years,

dark matter orchestrates the
construction of the universe.

[ rumbling ]

in these, the first battles
of the cosmos,

this constructive force
is the clear victor.

For the time being,
the dark matter has won.

Galaxies continue to form.

Clusters of galaxies
are getting bigger over time.

[ rumbling ]

narrator:
But dark matter's success in building up the universe

sets in motion
its potential downfall.

[ whooshing, rumbling ]

as the cosmic web evolves
into a more complex structure...

...Gaps form between
the filaments, the cosmic voids.

the cosmic voids formed because
other, more dense regions

of the universe gravitationally
stole material away from them.

So the dense parts
of the universe

accumulated more matter at the
expense of the less dense parts,

which then became voids.

Narrator: And lurking
in these voids, dark energy.

Since the dawn of time,

it's been waiting
for its opportunity.

Now it's preparing an offensive
that may help it

conquer the universe.

In the very earliest times,

the dark matter dominated
everything.

It was the big brother pushing
the little brother around.

But in the long run,

the dark energy is going
to overpower dark matter,

and so the relationship
is entirely flipped.

plait: We used to think
the fate of the cosmos itself

depended on dark matter,

and it turns out
that's not the case at all.

The fate of the universe
depends entirely on dark energy.

Narrator:
The long reign of dark matter

may be coming to an end.

Dark energy,
the great destroyer,

is hoping to take control.

This destructive force
has one overriding aim --

to tear the universe apart.

[ rumbling ]

[ rumbling ]

narrator:
Our universe is at war,

a relentless conflict
between dark forces.

For the first 9 billion years,
dark energy is subjugated.

Dark matter has the upper hand.

Frenk: When the universe
emerged from the big bang,

the dark energy played no role.

It was insignificant.

[ rumbling ]

narrator: But at some stage
in the 14 billion years

since the big bang,
these roles became reversed.

Dark energy came to be
the more powerful force.

The question was, when?

The answer came at the end
of the 20th century.

So it was an amazing
breakthrough, really important.

narrator: In 1999,

scientists measure
the expansion of the universe.

[ whooshing ]

what they find shocks them.

They expect the speed
of expansion to be decreasing.

In fact,
it's actually increasing

and getting faster all the time.

The data indicate that for about
the first 9 billion years,

it was slowing down.

But then,
in the past 5 billion years,

it started accelerating
faster and faster.

Narrator: Alex filippenko
was part of the team

that made this
explosive discovery.

Filippenko:
It befuddled us.

This isn't how nature
was supposed to be behaving,

and, in fact,
initially we thought

that there was something wrong

with either the observations
or the measurements.

I didn't believe it
for the longest time.

When the first data came out,
I'm like,

"nah, I don't believe this,
no way."

but it's in the data.

It's there.
You can't escape it.

This is as shocking as if you
held up a rock, let go of it,

and it went up into the air.

narrator:
Five billion years ago,

galaxies started moving apart
faster than before.

The question is, why?

What could be causing that?
Well, one thing is clear.

It must be getting some
extra energy from somewhere.

Narrator:
There is one main contender

for what may be supplying
this extra energy,

a force with repulsive gravity,

a force
that pushes things apart.

This is what astronomers
call dark energy.

It's this mysterious
repulsive force

that we know exists
in the universe,

and we have no idea what it is.

Narrator: Physicists may not
agree on what dark energy is,

but there is a consensus

on where this repulsive force
has the most influence --

in the regions between galaxies
and galaxy clusters,

the cosmic voids.

They're actually filled
to the brim with dark energy.

The first time dark energy
is really going to make

its mark in the universe
is going to be the time

when the first cosmic voids
begin to appear.

[ whooshing ]

sutter:
We see dark energy's effects throughout the universe.

But when we look into
the cosmic voids,

which are the most empty regions
of our universe,

this is where dark energy
is strongest.

narrator: Dark energy
is the repulsive force

pushing things apart.

It prefers the voids
where gravity is weak.

[ rumbling ]

bullock: These are areas where
there's a lot less dark matter,

and because the overall density
is low,

that's where the dark energy
starts to peek out

and can really drive
those voids to expand.

So the expansion and
acceleration of the universe

are driven by the dark energy
in those regions.

[ whooshing ]

narrator: Dark energy
pushes thing apart,

things that get in its way,

things like the cosmic web.

Dark matter and normal matter
are also in its path

and are bulldozed out
across the cosmos.

Slowly, but surely, the balance
between dark energy

and dark matter is changing.

[ whooshing ]

imagine you have
a giant swimming pool,

and at the very bottom
there's a puddle of water

with a splash of whiskey.

So you have sort of a strong
whiskey drink

down at the bottom of your pool.

But now you start dumping water
into your pool, no more whiskey,

and it begins to get diluted
and diluted,

and eventually, you just have
a swimming pool full of water,

with one shot of whiskey
mixed in.

That's not a very strong drink.

It's basically a water
swimming pool.

That's pretty much happening
with the dark energy.

At first,
it's a one-to-one mixture

of dark matter and dark energy.

But in the long run,
it's all dark energy

and pretty much
no dark matter left over.

narrator:
The forces of dark energy

are on an unstoppable March,

picking up more and more power

from the vast scale
of the cosmic voids.

Dark energy is intrinsically
very weak.

There's very little dark energy

and this repulsive effect
in every cubic centimeter.

But the universe is vast.

Space is big.

So cumulatively, all this
small amount of stuff

adds up to a very large amount,

and over a scale encompassing
the entire universe,

the dark energy dominates.

[ rumbling ]

narrator: But it's been
a very long process.

After the big bang,

dark matter dominates
for the first 9 billion years.

then, 5 billion years ago,

dark energy starts
to get the upper hand.

It causes the expansion
of the universe to accelerate

and the space in the voids
to grow more rapidly.

As the space expands, there's
more and more dark energy

because you have a bigger space.

It sort of creates itself
with the expansion of space.

Tegmark: Dark energy has
a sneaky way of taking over

because it causes the space

to stretch out
and get twice as big.

So now there's twice
as much dark energy.

Narrator: Dark energy
just can't stop pushing,

causing the empty space
of the voids

to continuously expand.

[ rumbling ]

as the universe expands
because of dark energy,

more and more dark energy
is being created.

Dark energy is
definitely gaining

the upper hand on dark matter.

It was always there, but it took
over compared to other stuff.

narrator: Eventually creating
enough energy to supercharge

the expansion of the universe.

this acceleration continues.

The universe is getting bigger
and bigger,

and it's all powered
by the forces of repulsion,

dark energy.

And for the universe,
that could be very bad news.

If that's the case, dark energy
may destroy the universe.

It will get stronger
and stronger

until it literally rips apart
the fabric of space-time.

[ rumbling ]

[ rumbling ]

narrator:
Dark matter and dark energy

have been battling each other
for 13.8 billion years.

For the first 9 billion years,
dark matter dominates.

Dark matter exerts
positive gravity,

but pulling everything together
leads to one inevitable outcome.

If the universe was totally
dominated by matter,

eventually our expansion
would slow down,

glide to a stop,
and then turn around

and collapse into
a small, dense state

from where it came from,
an event we call the big crunch.

Narrator:
During the big crunch,

gravity would play havoc
with the cosmos.

Galaxies
would be dragged together.

Stars and planets
would smash into each other.

The universe would collapse
in a blazing inferno

of superdense matter
and energy.

[ rumbling ]

fortunately, none of this
will probably happen.

scientists have now dismissed
the possibility of a big crunch.

sutter: We don't face that,
because we have a universe

filled with dark energy.

Dark energy is causing the
universe to do something else,

something it would prefer
not to do.

It is accelerating the expansion
of the universe.

Narrator:
As dark energy gets stronger,

it supercharges this expansion.

The presence of dark energy
is like a high-octane additive

into a gas tank,

where a car isn't
just coasting along.

It's boosting along,

and that's what's happening
with our universe.

Narrator: If that's correct,
this supercharged expansion

will continue
for the next few billion years.

The repulsive force of dark
energy will become invincible,

and such unbridled power
will come with a high cost.

I know of three ultimate
doomsday weapons.

One is galactus'
ultimate nullifier.

Another is the infinity stones
in the hands of thanos.

But those two are make-believe.

The real one is dark energy.

It's the real ultimate
universe destroyer.

narrator: The universe may pay
the ultimate price

for this ever-increasing
expansion driven by dark energy.

Carroll:
And if that continues forever,

the future of the universe
is very, very simple.

It continues to expand,
to accelerate.

Everything moves apart
from everything else,

and the universe
becomes empty.

There'll be nothing left

but cold, desolate,
empty space forever.

narrator:
Dark energy will finally win

the long war with dark matter.

But the result
won't be pretty.

This future universe
will be cold, dark, and empty.

Eventually it's going
to get so cold

that really nothing
can happen in it.

It's the big chill.

It gets colder and colder
and darker and darker.

Everything will fade out,
and though it began with a bang,

it's going to die in a whimper.

The universe flatlines
because of dark energy.

Narrator: The universe dies
in a big chill.

Galaxies are so far apart,

they're distant islands
in a sea of darkness.

Gradually,
the galaxies dies, too.

Star birth stops,
and the universe fades away.

Or maybe not.

There's another,
far more violent scenario.

Here, dark energy just goes
from strength to strength.

It could be that dark energy
is so strong

that it will multiply
upon itself

as the universe gets bigger.

This is a process that
we call phantom dark energy.

Narrator: Phantom energy is
dark energy on steroids.

It multiplies uncontrollably
in the voids,

tearing at the fabric
of the universe

in a process
called the big rip.

Dark energy is weird enough,
but imagine the possibility

that there is more and more
of it as time goes on,

and it's called phantom energy,
and in that case,

it would rip everything apart,
even black holes.

[ whooshing ]

it will start to rip apart
galaxies themselves...

[ rumbling ]

...Rip apart solar systems...

[ rumbling ]

...Rip apart people,

rip apart atoms...

Rip apart nuclei.

[ rumbling ]

until finally,
space itself is pulled apart.

[ whooshes ]

narrator:
The universe, as we know it,

will be destroyed,

but the big rip may not be
the end of everything.

there will be no normal matter
and no dark matter.

And with nothing
left to conquer,

phantom energy may use its
powers to become a creator,

triggering a rebirth.

[ bursting ]

eventually, when you get to this
ultimate stage of emptiness,

because of the phantom energy,

we're actually able to turn
the universe around

and get it
to collapse again,

and then go through
a series of bounces.

So we call it
the phantom bounce.

narrator:
With this phantom bounce,

universal life energy left
in this dead universe

starts to collapse.

Freese: And eventually
it becomes hotter and hotter

and denser and denser,
and then the fiery inferno

eventually pushes you back out
into another big bang,

and this just keeps
going on indefinitely.

narrator: So the destructive
and repulsive dark energy

spawns a force that becomes
the ultimate universe recycler.

The end state of our universe

would lead you back
into another cycle,

a whole new big bang
from the beginning.

[ bursting, rumbling ]

narrator: In the end,
dark energy may kill the cosmos,

or it may create a new one.

Dark energy is mysterious.

Dark energy is unknown.

Dark energy is going to do
whatever it feels like.

Maybe dark energy will go away.

Maybe dark energy will decay

and become a flood
of new matter and radiation.

Maybe dark matter
will get stronger.

We don't know.

narrator:
For now, we think dark energy

will determine the fate
of the universe.

But all of our evidence
is speculative.

What if we have it all wrong?

What if there is
no dark universe at all?

[ rumbling ]

narrator: A large part of
our understanding

of the universe's past,

present, and future is based
on educated guesswork

about two invisible forces --

[ crackling ]

...Dark matter and dark energy.

But it's pure speculation.

Perhaps dark matter
and dark energy don't exist.

There's not new stuff
in the universe.

Anything is possible.

Dark energy, in particular,

might not be real,
so maybe there's something else

that could be pushing
the universe apart.

Tremblay: So we could
absolutely be wrong

about dark energy
and dark matter.

Maybe they don't exist.

Maybe tomorrow we'll discover
that our understanding

was wrong all along.

Narrator:
That's an awful lot of maybes.

Let's add one more.

When it comes to finding
answers,

maybe we're looking
in the wrong place.

Filippenko: One possibility is
that there are other universes

out there pulling outward,
so to speak, on our universe.

That might be the answer.

But most theoretical physicists
and astrophysicists

these days think
that dark energy is real

because that seems to be
the simplest explanation

for a wide variety
of observations.

narrator: No one really knows
what dark energy is made of.

Maybe the answer lies
in the past.

The best theory for dark energy
we have right now

is the simplest one
and the oldest one,

and that's the idea that
it's a cosmological constant.

narrator: Albert einstein
came up with the idea

of a cosmological constant
in 1917.

He suggested that space
has its own energy,

energy that can affect
the way the universe expands.

when edwin hubble proved
the universe is expanding,

einstein thought
the cosmological constant

was his biggest blunder.

But observations that the
expansion of the universe

is accelerating

reveal einstein
was right all along.

Filippenko: Well, here we are.
We've reintroduced the idea.

So einstein's biggest blunder
may have actually

conceptually been
his greatest triumph.

narrator:
But to understand the true nature of the dark universe,

we may need to re-evaluate what
we think we know about gravity.

[ rumbling ]

when we're trying to understand
dark energy and dark matter,

there's a chance that just our
fundamental theories of gravity

are wrong, that general
relativity isn't quite right.

Narrator: Einstein's theory
of general relativity

explains how gravity works,

how stars orbit in galaxies,

and planets orbit stars.

some scientists wonder
if altering this theory

will help us understand
the dark universe.

Tremblay: So you need not
absolutely believe

that there is something
actually called dark matter.

You need to only understand
that there is something

in the universe
which behaves like dark matter.

For example, you could
effectively mimic the behavior

of dark matter by modifying
our current theory of gravity.

Narrator:
But successfully modifying

einstein's theories on gravity
is a big challenge.

Einstein's equations
are very robust.

You don't fluff around
with einstein with impunity.

[ rumbling ]

narrator: For decades,
theoretical physicists

have toyed
with einstein's equations,

looking for ways to explain
dark matter and dark energy,

or make them go away.

As yet, no one has managed.

The dark universe persists.

[ whooshing ]

I think that the best
description of the observations

we have today
is that dark matter exists.

It's out there,
as well as dark energy.

I think dark energy exists.

I think dark energy is real,

but I must admit that sometimes,
at 3 o'clock in the morning,

I wake up screaming,
worried that, in fact,

we've settled
on the wrong answer,

and that in
a couple hundred years,

they're going to be
laughing at us.

narrator: Until then, our
observations tell us the battle

between dark matter and dark
energy has shaped the universe.

dark matter dictated the past,

built the galaxies,
the stars, and the planets.

dark energy will determine
its future,

potentially tearing
the universe apart.

Just because we can't see dark
matter and dark energy directly

doesn't mean they have not
had a profound effect

on the evolution
of the entire universe.

The dark universe was there at
the beginning of the universe,

shaping it, and actually
creating the conditions

for us to be here, and it's
taken over the universe.

And, eventually, it may well
destroy the universe.

narrator:
Our universe may be dominated

by the long struggle between
dark matter and dark energy.

But all of this conflict
has led to a creative outcome,

an outcome for which
we should all be grateful.

The name dark matter suggests
that it's something nefarious

and somehow bad for us,
but actually it's turned out

that dark matter
is very much our friend.

Because if it weren't
for the dark matter,

we wouldn't be here.

[ rumbling ]

thaller:
There's a wonderful irony

to calling it
the dark universe,

because now we're actually
beginning to shed light

on how the universe began,

how the largest structures
in the universe evolved.

We wouldn't be here
without this dark universe.

It's not dark at all.

It's shedding light
on our own reality.

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