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00:00Across the universe, an endless war rages, a bitter struggle between invisible forces.
00:15We've seen this cosmic battle go on for the past 14 billion years.
00:20Like two navies fighting it out in the ocean of space time.
00:25Dark matter and dark energy, battling for control of the universe.
00:32They've shaped the entire history of the universe.
00:35They're not about to stop now.
00:37Shadowy forces dictating our past, our present, and our future.
00:44The dark universe is kind of a puppet master behind the scenes, guiding the universe that
00:48we can see.
00:50It's taken over the universe, and eventually, it may well destroy the universe.
01:17As we look out across the universe, we see nebulas, stars, and planets, all constructed
01:29from visible matter.
01:32But what we see is just a small part of the cosmos.
01:38The rest is invisible, unknown.
01:43The dark universe.
01:47Dark universe is a very common phrase to describe our universe because it turns out
01:52most of our cosmos is dark.
01:56Dark not as in night, dark as in doesn't interact with light, and dark as in we don't
02:03really understand it.
02:05All the objects in our daily experience, our bodies, the air, the chair that you're sitting
02:10in, the planets and stars, all of that only adds up to 5% of the universe.
02:19It's the other 95% that is the dark stuff, the dark universe.
02:27The dark universe is made of two forces, dark matter and dark energy.
02:36In a way, the dark matter and dark energy sort of oppose each other.
02:44Dark matter has positive gravity that pulls things together, and dark energy has repulsive
02:49gravity that pushes things apart.
02:53To encapsulate that in one sentence, dark matter is attractive, dark energy is repulsive.
02:59Since the dawn of time, two forces slugging it out for control of the universe.
03:05Dark matter and dark energy are locked in this epic struggle.
03:08The dark matter is trying to bring things together, the dark energy is trying to drive
03:12everything apart.
03:15So it's basically a battle, who's going to win?
03:22It's a struggle that started 13.8 billion years ago in the cosmic furnace of the Big
03:28Bang.
03:32The infant universe was a super hot ball of intense radiation, but suddenly it started
03:39to transform.
03:40It cooled and expanded, triggering the birth of the regular universe and the dark universe.
03:51Scientists believe that both dark matter and dark energy formed in the first moments of
03:57the Big Bang.
03:58It would have formed probably just fractions of a second after the Big Bang, around the
04:04time that normal matter formed.
04:06And the processes that created the normal matter we know all about, similar processes
04:11probably created the dark matter.
04:15During these first microseconds, the universe was a hot, dense ball of matter and energy.
04:22They're two sides of the same coin.
04:25Matter can convert into energy, and energy can convert directly into matter.
04:33Visible particles of regular matter, forming atoms, planets, stars, and us, and other particles,
04:42they are invisible.
04:44They are dark matter.
04:46One of the big mysteries that we as astronomers have to solve is what this dark matter is.
04:52We just don't know.
04:55The idea of a type of matter that you can't see and that acts differently than normal
05:00matter is sort of out there.
05:02It's sort of weird.
05:05But the dark universe does leave clues.
05:08It's like a crime scene.
05:10You know that a crime has been committed, even though you don't know the perpetrator
05:14yet.
05:15We see the hints.
05:17We see the signs.
05:18We see the signals that something funny is happening in our universe, even though we
05:23don't know exactly what's causing it.
05:27There are several contenders for what dark matter could be.
05:30It could be normal matter that we just don't see, or it could be some sort of exotic matter,
05:36a particle of some sort that we haven't detected yet.
05:39It could be a species of particle, like an electron, like a proton, like a quark, but
05:48a special kind that doesn't interact with light.
05:52One of the favorite possible models of dark matter are WIMPs, weakly interacting massive
05:57particles.
05:58WIMPs may not interact strongly with other matter or light, but they do exert a gravitational
06:05pull.
06:09So they're the best candidates for the particles of dark matter that formed in the Big Bang.
06:17The early universe was intensely hot and dense, full of new particles of both visible
06:23matter and invisible dark matter.
06:28But another force was present, dark energy.
06:34Dark energy has existed since the time of the Big Bang.
06:38Dark energy was always there.
06:39We don't know how much of it there was, but it's possible that the same amount of dark
06:44energy was always there.
06:48Our understanding of what that dark energy is, is very limited.
06:53If you were to ask a roomful of 10 theoretical physicists on the nature of dark energy, you'd
06:58get about 12 different answers.
07:01We're not sure what dark energy is.
07:05Dark energy is just a fancy name for our ignorance.
07:09Dark energy is nothing more than a placeholder name for this enormous gap in our understanding
07:15of how the cosmos works.
07:16But we don't understand it at all.
07:19That's true.
07:23One potential answer to what dark energy actually is may be found in so-called empty space.
07:31Could this be the source of dark energy?
07:35We used to think of space as just boring emptiness, but now I think it's healthier to think of
07:41space as a kind of substance.
07:44A substance that carries a strange type of energy.
07:48Every small region of space has a little bit of energy in it, just associated with the
07:53vacuum itself.
07:55Vacuum energy is the idea that vacuum isn't empty, that there's something there, there's
08:00an energy in it with a kind of anti-gravity.
08:07Perhaps this vacuum energy that pushes against gravity is the mysterious dark energy.
08:16We simply don't know.
08:20We're not sure that the dark energy is the vacuum energy.
08:23It could be a new type of energy that permeates all of space.
08:28This is what we're trying to measure now.
08:31Dark energy and dark matter, forged in the intense heat of the Big Bang.
08:37Opposing forces, one attractive, one repulsive.
08:42But together, over 13.8 billion years, they will shape the history of the universe.
08:58The story of the universe is dominated by two powerful opposing forces.
09:10Dark matter and dark energy.
09:14For 13.8 billion years, they've battled it out for control of the cosmos.
09:20Dark matter and dark energy are out there and they've shaped the entire history of the
09:24universe.
09:25The universe is actually the balance between dark forces.
09:28Dark matter is trying to draw everything together and dark energy is trying to rip everything
09:32apart.
09:35After the Big Bang, the infant universe was small, intensely hot, and intensely dense.
09:44Dark matter, the force bent on bringing things together, thrived.
09:50In this compressed space, dark energy, the force trying to drive things apart, had no
09:56room to act.
09:59When things were closer together, the density of matter and radiation was bigger.
10:04So big that the dark energy didn't matter.
10:07The environment was also tough for normal matter.
10:12It was so hot, intense radiation prevented visible matter from bunching together to form
10:20If any normal matter tried to clump together through gravity or some other force, this
10:24energy would just basically blast it apart.
10:27In the very early universe, when our universe was a lot smaller and a lot hotter and a lot
10:31denser, matter, normal matter, tried to collect together, wanted to join the party.
10:38But it was prevented from doing so because there was also radiation that would throw
10:43it out.
10:46Unable to stick together, normal, visible matter sped out across the infant cosmos in
10:52a blizzard of particles.
10:55But then dark matter, the force that brings things together, intervened.
11:03Dark matter doesn't talk to radiation, doesn't talk to light, can do whatever it wants.
11:08It starts clumping together.
11:12Radiation pushes normal matter apart, stopping it from forming dense regions.
11:18But photons simply pass straight through the dark matter, allowing it to clump and fall
11:24into dense pockets or wells.
11:28The dark matter begins to clump together gravitationally.
11:32And this means that the matter is going to fall into those dark matter wells.
11:38Over time, more and more regular matter is pulled into the dark matter wells.
11:44The regions that have a little bit more stuff, gravity makes them bigger, and the regions
11:47that have less stuff, those expand more.
11:50So you have little pockets of slight extra matter, have more and more and more matter
11:55over time.
11:58Gravity-rich pockets of dark matter pull particles of regular matter together.
12:05Gradually, they form giant clouds of hydrogen and helium gas.
12:12Dark matter has laid the foundation stones of the cosmos.
12:16This force may be dark, but it's highly creative.
12:24And now, 180 million years after the Big Bang, everything is in place for the next dark matter
12:32construction milestone, the creation of stars.
12:38We know stars in the very early universe at the edge of time had to form from the collapse
12:43of gas clouds under their own gravity.
12:48But there's a problem.
12:50The clouds of hydrogen in the infant universe can't collapse.
12:55And no collapse means no stars.
12:59The gas in the early universe has a lot of pressure, and this pressure keeps it from
13:04collapsing.
13:05The dark matter doesn't experience that kind of pressure.
13:07So the dark matter can clump up and make sites for structure formation.
13:12So dark matter comes to the rescue, creating regions of higher gravity, dragging in hydrogen
13:19gas, forcing the clouds to get denser and denser, creating the conditions for collapse
13:27and then creation.
13:36So it's only when the gravity of the dark matter overwhelms the pressure of gases that
13:42the gases can collapse and turn into stars.
13:45As soon as the gas cools down, it can fall into those potential wells that the dark matter
13:49created, almost like little nurseries for stars, and they start forming in earnest.
13:58Dark matter provides a boost of gravity to kick-start hydrogen into constructing the
14:03first stars, stars that are the seeds of the first galaxies.
14:09So it's dark matter that would have coalesced in the early universe and grown from there.
14:16And then the luminous component of the universe, the things that we think of as being the universe
14:19itself, like stars and galaxies, would have just been along for the ride.
14:24Without the presence of dark matter to seed structures, there wasn't enough time in the
14:31early universe to form galaxies, which means you and me have to thank dark matter for our
14:38existence.
14:39And dark matter now begins a much more ambitious architectural project to shape the entire
14:48universe itself, to build the biggest structure ever constructed, the cosmic web.
15:10The war between dark matter and dark energy has been raging since the birth of time.
15:19But in the early years, it's a one-sided contest.
15:24In the early universe, the only thing that really mattered was the dark matter and the
15:27normal matter.
15:30Dark matter, the force that brings things together, is in the driver's seat.
15:41Dark energy, the force that pulls things apart, is the underdog.
15:47We've seen this cosmic battle go on for the past 14 billion years.
15:52Fortunately for us, the dark energy got off to a slow start.
15:57Meanwhile, dark matter is busy at work, building the universe.
16:03Not only does it trigger the birth of the first stars, it embarks on an even more formidable
16:10construction project, the cosmic web.
16:16There's this large-scale structure of filaments that galaxies seem to form on, and that's
16:23what we call the cosmic web.
16:25And we can trace the formation of this cosmic web all the way back to the early universe.
16:31Now this is such a huge structure, we don't think there's time in the universe for matter's
16:35gravity alone to do this.
16:37There must have been an underlying scaffold of dark matter.
16:41The dark matter started forming into these filaments, and when the universe cooled enough,
16:46normal matter could start to stream in to this gravitational attraction of the dark
16:49matter.
16:50And that became the scaffolding on which this large-scale structure was built.
16:56The filaments of dark matter join together, drawing in more and more hydrogen gas.
17:02Dense clouds of gas build up at the junctions of the filaments, the point where gravity
17:08is at its strongest.
17:13Slowly and surely, a familiar-looking structure starts to take shape.
17:19If you've ever gone outside and you can see a spider web covered in dew, that's kind of
17:25like what happened with the universe.
17:27In this case, the spider web is the structure of the dark matter, it's all of these filaments,
17:32and the moisture in the air is what condenses around them, just like the normal matter fell
17:36into the dark matter web to form these gigantic structures in the universe.
17:42Dark matter, the universe's master builder, succeeds in stitching together a cosmic web.
17:50This will be the framework for the entire universe.
17:54And so it is dark matter that would choreograph and sculpt the shape of the universe itself.
18:00One of the amazing things about dark matter is, without it, we wouldn't be here.
18:04It's hard to imagine how you could have structure in the universe without dark matter.
18:11And then galactic clusters form at the junctions of the filaments.
18:15Slowly but surely, the universe begins to take shape.
18:20When we look at this structure over a cosmic scale, we see that it looks kind of like a
18:25sponge.
18:26You see voids with galaxies all over the edges of them.
18:29That is the structure that was formed by the dark matter in the early universe.
18:35Dark matter is the thing that enabled, that provided enough gravity for the initial seeds
18:41of structure formation to coalesce, for galaxies themselves to form.
18:46And of course, without galaxies, there are no stars, and there's no planets, and there's
18:51no us.
18:53For nine billion years, dark matter orchestrates the construction of the universe.
19:00In these, the first battles of the cosmos, this constructive force is the clear victor.
19:07For the time being, the dark matter has won.
19:10Galaxies continue to form.
19:12Clusters of galaxies are getting bigger over time.
19:17But dark matter's success in building up the universe sets in motion its potential downfall.
19:28As the cosmic web evolves into a more complex structure, gaps form between the filaments,
19:37the cosmic voids.
19:41The cosmic voids formed because other, more dense regions of the universe gravitationally
19:47stole material away from them.
19:50So the dense parts of the universe accumulated more matter at the expense of the less dense
19:56parts, which then became voids.
20:00And lurking in these voids?
20:03Dark energy.
20:04Since the dawn of time, it's been waiting for its opportunity.
20:10Now it's preparing an offensive that may help it conquer the universe.
20:16In the very earliest times, the dark matter dominated everything.
20:19It was the big brother pushing the little brother around.
20:23But in the long run, the dark energy is going to overpower dark matter.
20:27And so the relationship is entirely flipped.
20:31We used to think the fate of the cosmos itself depended on dark matter, and it turns out
20:36that's not the case at all.
20:38The fate of the universe depends entirely on dark energy.
20:42The long reign of dark matter may be coming to an end.
20:46Dark energy, the great destroyer, is hoping to take control.
20:52Its destructive force has one overriding aim.
20:56To tear the universe apart.
21:10Our universe is at war.
21:13A relentless conflict between dark forces.
21:17For the first nine billion years, dark energy is subjugated.
21:23Dark matter has the upper hand.
21:28When the universe emerged from the Big Bang, the dark energy played no role.
21:32It was insignificant.
21:35But at some stage in the 14 billion years since the Big Bang, these roles became reversed.
21:43Dark energy came to be the more powerful force.
21:47The question was, when?
21:51The answer came at the end of the 20th century.
21:54So it was an amazing breakthrough, really important.
21:59In 1999, scientists measure the expansion of the universe.
22:05What they find shocks them.
22:08They expect the speed of expansion to be decreasing.
22:13In fact, it's actually increasing and getting faster all the time.
22:20The data indicate that for about the first nine billion years, it was slowing down.
22:24But then in the past five billion years, it started accelerating faster and faster.
22:31Alex Filippenko was part of the team that made this explosive discovery.
22:37It befuddled us.
22:39This isn't how nature was supposed to be behaving.
22:42And in fact, initially, we thought that there was something wrong with either the observations
22:47or the measurements.
22:48I didn't believe it for the longest time.
22:50When the first data came out, I'm like, nah, I don't believe this, no way.
22:54But it's in the data, it's there, you can't escape it.
22:58This is as shocking as if you held up a rock, let go of it, and it went up into the air.
23:06Five billion years ago, galaxies started moving apart faster than before.
23:11The question is why?
23:14What could be causing that?
23:16Well, one thing is clear, it must be getting some extra energy from somewhere.
23:21There is one main contender for what may be supplying this extra energy, a force with
23:27repulsive gravity, a force that pushes things apart.
23:33This is what astronomers call dark energy.
23:35It's this mysterious repulsive force that we know exists in the universe, and we have
23:40no idea what it is.
23:43Physicists may not agree on what dark energy is, but there is a consensus on where this
23:49repulsive force has the most influence, in the regions between galaxies and galaxy clusters,
23:58the cosmic voids.
24:00They're actually filled to the brim with dark energy.
24:04The first time dark energy is really going to make its mark in the universe is going
24:08to be the time when the first cosmic voids begin to appear.
24:14We see dark energy's effects throughout the universe, but when we look into the cosmic
24:20voids, which are the most empty regions of our universe, this is where dark energy is
24:26strongest.
24:30Dark energy is the repulsive force pushing things apart.
24:34It prefers the voids, where gravity is weak.
24:39These are areas where there's a lot less dark matter, and because the overall density is
24:44low, that's where the dark energy starts to peak out and can really drive those voids
24:48to expand.
24:49So the expansion and acceleration of the universe are driven by the dark energy in those regions.
24:56Dark energy pushes things apart.
25:00Things that get in its way.
25:03Things like the cosmic web.
25:07Dark matter and normal matter are also in its path and are bulldozed out across the
25:14cosmos.
25:15Slowly, but surely, the balance between dark energy and dark matter is changing.
25:25Imagine you have a giant swimming pool, and at the very bottom there's a puddle of water
25:29with a splash of whiskey, so you have sort of a strong whiskey drink down at the bottom
25:33of your pool.
25:34But now you start dumping water into your pool, no more whiskey, and it begins to get
25:38diluted and diluted, and eventually you just have a swimming pool full of water with one
25:43shot of whiskey mixed in.
25:44That's not a very strong drink.
25:46It's basically a water swimming pool.
25:48That's pretty much what's happening with the dark energy.
25:50At first it's a one-to-one mixture of dark matter and dark energy, but in the long run
25:55it's all dark energy and pretty much no dark matter left over.
26:00The forces of dark energy are on an unstoppable march, picking up more and more power from
26:07the vast scale of the cosmic voids.
26:11Dark energy is intrinsically very weak.
26:14There's very little dark energy and this repulsive effect in every cubic centimeter.
26:20But the universe is vast, space is big, so cumulatively all this small amount of stuff
26:28adds up to a very large amount, and over a scale encompassing the entire universe, the
26:34dark energy dominates.
26:38But it's been a very long process.
26:41After the Big Bang, dark matter dominates for the first nine billion years.
26:47Then, five billion years ago, dark energy starts to get the upper hand.
26:54It causes the expansion of the universe to accelerate, and the space in the voids to
26:59grow more rapidly.
27:01As the space expands, there's more and more dark energy, because you have a bigger space.
27:07It sort of creates itself with the expansion of space.
27:12Dark energy has a sneaky way of taking over, because it causes the space to stretch out
27:16and get twice as big, so now there's twice as much dark energy.
27:22Dark energy just can't stop pushing, causing the empty space of the voids to continuously
27:28expand.
27:32As the universe expands because of dark energy, more and more dark energy is being created.
27:38Dark energy is definitely gaining the upper hand on dark matter.
27:41It was always there, but it took over compared to other stuff.
27:48Eventually, creating enough energy to supercharge the expansion of the universe.
27:56This acceleration continues.
27:58The universe is getting bigger and bigger, and it's all powered by the forces of repulsion.
28:04Dark energy.
28:05And for the universe, that could be very bad news.
28:10If that's the case, dark energy may destroy the universe.
28:15It will get stronger and stronger until it literally rips apart the fabric of spacetime.
28:30Dark matter and dark energy have been battling each other for 13.8 billion years.
28:36For the first 9 billion years, dark matter dominates.
28:41Dark matter exerts positive gravity, but pulling everything together leads to one inevitable
28:48outcome.
28:49If the universe was totally dominated by matter, eventually our expansion would slow down,
28:57glide to a stop, and then turn around and collapse into a small, dense state from where
29:04it came from, an event we call the Big Crunch.
29:09During the Big Crunch, gravity would play havoc with the cosmos.
29:15Galaxies would be dragged together.
29:19Stars and planets would smash into each other.
29:22The universe would collapse in a blazing inferno of super-dense matter and energy.
29:32Fortunately, none of this will probably happen.
29:39Scientists have now dismissed the possibility of a Big Crunch.
29:46We don't face that because we have a universe filled with dark energy.
29:51Dark energy is causing the universe to do something else, something it would prefer
29:55not to do.
29:57It is accelerating the expansion of the universe.
30:01As dark energy gets stronger, it supercharges this expansion.
30:06The presence of dark energy is like a high-octane additive into a gas tank, where a car isn't
30:16just coasting along, it's boosting along, and that's what's happening with our universe.
30:23If that's correct, this supercharged expansion will continue for the next few billion years.
30:30The repulsive force of dark energy will become invincible, and such unbridled power will
30:36come with a high cost.
30:40I know of three ultimate doomsday weapons.
30:44One is Galactus' ultimate nullifier.
30:47Another is the Infinity Stones in the hands of Sophanos.
30:51But those two are make-believe.
30:53The real one is dark energy.
30:55It's the real ultimate universe destroyer.
31:01The universe may pay the ultimate price for this ever-increasing expansion driven by dark
31:07energy.
31:08If that continues forever, the future of the universe is very, very simple.
31:12It continues to expand, to accelerate, everything moves apart from everything else, and the
31:17universe becomes empty.
31:20There'll be nothing left but cold, desolate, empty space forever.
31:28Dark energy will finally win the long war with dark matter.
31:32But the result won't be pretty.
31:35This future universe will be cold, dark, and empty.
31:41Eventually it's going to get so cold that really nothing can happen in it.
31:45It's the big chill.
31:46It gets colder and colder and darker and darker.
31:50Everything will fade out, and though it began with a bang, it's going to die in a whimper.
31:55The universe flatlines because of dark energy.
31:59The universe dies in a big chill.
32:03Galaxies are so far apart, they're distant islands in a sea of darkness.
32:08Gradually, the galaxies die too.
32:12Our birth stops, and the universe fades away.
32:19Or maybe not.
32:21There's another, far more violent scenario.
32:24Here, dark energy just goes from strength to strength.
32:30It could be that dark energy is so strong that it will multiply upon itself as the universe
32:39gets bigger.
32:40This is a process that we call phantom dark energy.
32:45Phantom energy is dark energy on steroids.
32:50It multiplies uncontrollably in the voids, tearing at the fabric of the universe in a
32:56process called the big rip.
33:01Dark energy is weird enough, but imagine the possibility that there is more and more of
33:06it as time goes on, and it's called phantom energy.
33:12And in that case, it would rip everything apart, even black holes.
33:17It will start to rip apart galaxies themselves, rip apart solar systems, rip apart people,
33:33rip apart atoms, rip apart nuclei, until finally space itself is pulled apart.
33:45The universe as we know it will be destroyed, but the big rip may not be the end of everything.
33:56There will be no normal matter, and no dark matter.
34:01And with nothing left to conquer, phantom energy may use its powers to become a creator,
34:07triggering a rebirth.
34:09Eventually, when you get to this ultimate stage of emptiness, because of the phantom
34:16energy, we're actually able to turn the universe around and get it to collapse again, and then
34:22go through a series of bounces.
34:24So we call it the phantom bounce.
34:27With this phantom bounce, all energy left in this dead universe starts to collapse.
34:34And eventually it becomes hotter and hotter and denser and denser, and then the fiery
34:38inferno eventually pushes you back out into another big bang.
34:43And this just keeps going on indefinitely.
34:47So the destructive and repulsive dark energy spawns a force that becomes the ultimate universe
34:53recycler.
34:56The end state of our universe would lead you back into another cycle, a whole new big bang
35:01from the beginning.
35:08In the end, dark energy may kill the cosmos, or it may create a new one.
35:15Dark energy is mysterious.
35:17Dark energy is unknown.
35:18Dark energy is going to do whatever it feels like.
35:22Maybe dark energy will go away.
35:24Maybe dark energy will decay and become a flood of new matter and radiation.
35:29Maybe dark matter will get stronger.
35:31We don't know.
35:36For now, we think dark energy will determine the fate of the universe.
35:42But all of our evidence is speculative.
35:46What if we have it all wrong?
35:48What if there is no dark universe at all?
36:05A large part of our understanding of the universe's past, present and future is based on educated
36:11guesswork about two invisible forces.
36:17Dark matter and dark energy.
36:21But it's pure speculation.
36:24Perhaps dark matter and dark energy don't exist.
36:28There's not new stuff in the universe.
36:31Anything is possible.
36:32Dark energy in particular might not be real.
36:35So maybe there's something else that could be pushing the universe apart.
36:38So we could absolutely be wrong about dark energy and dark matter.
36:42Maybe they don't exist.
36:44Maybe tomorrow we'll discover that our understanding of it was wrong all along.
36:50That's an awful lot of maybes.
36:52Let's add one more.
36:54When it comes to finding answers, maybe we're looking in the wrong place.
37:01One possibility is that there are other universes out there pulling outward, so to speak, on
37:08our universe.
37:09That might be the answer.
37:11Most theoretical physicists and astrophysicists these days think that dark energy is real
37:16because that seems to be the simplest explanation for a wide variety of observations.
37:24No one really knows what dark energy is made of.
37:28Maybe the answer lies in the past.
37:31The best theory for dark energy we have right now is the simplest one and the oldest one.
37:36And that's the idea that it's a cosmological constant.
37:40Albert Einstein came up with the idea of a cosmological constant in 1917.
37:47He suggested that space has its own energy, energy that can affect the way the universe
37:53expands.
37:57When Edwin Hubble proved the universe is expanding, Einstein thought the cosmological constant
38:03was his biggest blunder.
38:05Observations that the expansion of the universe is accelerating reveal Einstein was right
38:12all along.
38:13Well, here we are.
38:16We've reintroduced the idea.
38:18So Einstein's biggest blunder may have actually conceptually been his greatest triumph.
38:28But to understand the true nature of the dark universe, we may need to reevaluate what we
38:33think we know about gravity.
38:37When we're trying to understand dark energy and dark matter, there's a chance that just
38:42our fundamental theories of gravity are wrong, that general relativity isn't quite right.
38:49Einstein's theory of general relativity explains how gravity works, how stars orbit in galaxies
38:56and planets orbit stars.
39:00Some scientists wonder if altering this theory will help us understand the dark universe.
39:07So you need not absolutely believe that there is something actually called dark matter.
39:13You need only understand that there is something in the universe which behaves like dark matter.
39:18For example, you could effectively mimic the behavior of dark matter by modifying our current
39:23theory of gravity.
39:25But successfully modifying Einstein's theories on gravity is a big challenge.
39:32Einstein's equations are very robust.
39:34You don't faff around with Einstein with impunity.
39:40For decades, theoretical physicists have toyed with Einstein's equations, looking for ways
39:46to explain dark matter and dark energy or make them go away.
39:52As yet, no one has managed.
39:54The dark universe persists.
39:58I think that the best description of the observations we have today is that dark matter exists.
40:04It's out there.
40:06As well as dark energy.
40:07I think dark energy exists.
40:08I think dark energy is real, but I must admit that sometimes at three o'clock in the morning
40:13I wake up screaming, worried that in fact we've settled on the wrong answer and that
40:20in a couple of hundred years they're going to be laughing at us.
40:27Until then, our observations tell us the battle between dark matter and dark energy has shaped
40:33the universe.
40:38Dark matter dictated the past, built the galaxies, the stars, and the planets.
40:46Dark energy will determine its future, potentially tearing the universe apart.
40:53Just because we can't see dark matter and dark energy directly doesn't mean they have
40:57not had a profound effect on the evolution of the entire universe.
41:01The dark universe was there at the beginning of the universe, shaping it, and actually
41:06creating the conditions for us to be here, and it's taken over the universe, and eventually
41:12it may well destroy the universe.
41:16Our universe may be dominated by the long struggle between dark matter and dark energy,
41:22but all of this conflict has led to a creative outcome, an outcome for which we should all
41:29be grateful.
41:30The name dark matter suggests that it's something nefarious and somehow bad for us,
41:36but actually it's turned out that dark matter is very much our friend, because if it weren't
41:40for the dark matter, we wouldn't be here.
41:45There's a wonderful irony to calling it the dark universe, because now we're actually
41:49beginning to shed light on how the universe began, how the largest structures in the universe
41:53evolved.
41:54We wouldn't be here without this dark universe.
41:56It's not dark at all, it's shedding light on our own reality.