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In 2016, astronomers make an extraordinary discovery out in deep

space. A cosmic megastructure, one billion light years long.

The Great Wall.

You could put 10,000 of our galaxies end to end, and that's how big the Great

Wall is.

Investigators are now racing to decode what shapes the most massive and

mysterious structure in the universe.

Something, some force or process caused the Great Wall to be sculpted in this

really particular way.

Can an alien-like creature reveal how the Great Wall fits into the cosmos?

This may be our best model of how the cosmic web was built.

And what strange force will define its future?

It too shall pass.

It's not going to last forever.

Find out.

we dive into the Great Wall's fiery birth and hunt for the weird cosmic glue

that holds it together to reveal how this megastructure rewrites our

understanding of the universe on an epic scale.

Our universe is vast.

Astronomers estimate that it measures 93 billion light years across, and that it

contains hundreds of billions of galaxies.

From Earth, it looks like these galaxies are entirely separate from one another,

solitary islands adrift in the expanse of space.

Almost all of the universe is empty. It's just cold and dark.

We have telescopes that are looking farther and farther out, and there seems

to be just more and more space between the galaxies, more darkness.

But astronomers today are finding patterns among the stars.

This evidence shows that many galaxies share a mysterious connection.

In January 2016,

astronomers spot something huge.

830 visible galaxies, trillions of stars,

woven into what appears to be a single flattened structure.

One billion light years tall and almost as wide.

This wall of galaxies is the most massive structure in the universe.

Astronomers name it after the baryon oscillation spectroscopic survey that

makes the discovery.

The BOSS Great Wall.

What is this strange, giant structure?

The special telescope that finds the Great Wall has a unique way to hunt for

clues.

This is the Sloan Digital Sky Survey.

or SDSS for short.

SDSS works differently to a regular telescope.

Instead of astronomers saying, hey, I'm interested in that object right there in

the sky and pointing the telescope there and looking at it, the Sloan Digital Sky

Survey tiles the sky.

It's short of squat and wide, and it has a wide field of view. So it sees a lot

more of the sky in one image than a typical telescope.

So you can create one big super image of the sky from it.

A quirk of physics allows this telescope to draw a special color-coded map that

plots the positions of objects in the universe with pinpoint accuracy.

This is how it works.

As our universe expands, it stretches the fabric of space.

and forces objects further and further apart.

This stretches the light that these objects emit and shifts its wavelength

and color to the red end of the spectrum.

The further away an object is from our telescopes, the stronger this redshift

will be.

SDSS can measure this redshift of distant galaxies to map the cosmos in

three dimensions.

The SDSS data reveals that the Great Wall is a clearly defined physical

structure.

It is deep as well as long.

The body of the Great Wall consists of the densest concentration of galaxies

ever recorded.

And this makes it the most massive structure in the universe that

astronomers have ever seen.

Why is it here?

How did it form?

Astronomers believe that the answer lies in the key building blocks of the Great

Wall. Its trillions of stars.

A star is a giant nuclear reactor that fuses atoms of hydrogen to produce light

and heat.

From one end of the universe to the other, all the light we see out there is

produced by the same simple thing, nuclear fusion. And that's true close by

with our sun and in the most distant reaches of the universe. It's our beacon

in the darkness.

Huge gas clouds, mostly made of hydrogen.

give birth to stars and the galaxies they live in.

These massive concentrations of gas explode into balls of nuclear fusion

under the right conditions.

We think of a star as this immensely hot furnace, and it is, but in order to

create one, you actually counter-intuitively need really cold gas

that collapses under its own gravity and grows and eventually becomes massive

enough to spark internal fusion.

It takes a vast amount of collapsing gas to make the huge number of stars in the

Great Wall.

The Great Wall is not only the most massive structure ever seen, it is also

the greatest concentration of hydrogen yet found in the universe.

The mass of the Great Wall is almost unbelievable.

If our galaxy, our massive, giant galaxy, were, say, the mass of a stick

of butter, the Great Wall would be the mass of a car.

The Great Wall owes its genesis to a super-huge cloud, made mostly of

hydrogen, that clumps together in one part of space.

How does so much gas end up dumped in this part of the universe?

A clue lies in the data that special satellites, like NASA's WMAP probe, beam

back to Earth.

The data draws this extraordinary image.

It is a picture of the universe when it is just 380,000 years old.

It shows the spread and concentration of ancient microwave energy.

Astronomers call this the cosmic microwave background or CMB.

The cosmic microwave background is literally what it says.

It is a background glow of the universe in the wavelengths of microwaves. What

it really is, is sort of the cooling off of the fireball of the Big Bang itself.

It is a snapshot of the universe when it was only a few hundred thousand years

old.

The CMB captures the moment after the Big Bang.

when the universe cools just enough for the first atoms of gas to form.

The image shows that the concentrations of this matter in the early universe

varies from place to place.

We see that it's not perfectly smooth.

There are places in the universe where there's a little bit more matter and

places where there's a little bit less.

Astronomers believe that patches of the early universe with the highest

concentrations of gas

eventually evolve into structures like the Great Wall.

But what triggers this unevenness in the first place?

Astronomers believe that the answer lies at the moment of the universe's

creation.

At a time when everything in existence fits into a space far smaller than an

atom.

The tiniest sliver of a fraction of a second after the Big Bang, one ten

trillion trillion trillionth of a second, the universe underwent an

inflationary period, where it went from about the size of a proton, a subatomic

particle, to roughly the size of a grain of sand.

Now, I know that doesn't sound like much, but you've got to realize this is

a factor of a million, billion, billion times.

This is a huge expansion.

This moment of inflation sows the seed that grows into the Great Wall.

Trillionths of a second after the Big Bang, the cosmos is a ball of energy

where weirdness rules.

Minute random fluctuations in this super hot soup create a pattern of hotspots on

subatomic scales As the universe expands

it stretches out the tiny hotspots over a cosmic scale

Energy condenses into matter which provides the

seeds for huge cosmic structures

like the clouds of hydrogen that feed the galaxies in the Great Wall.

The inflation of the universe pulled parts of the universe that were close

together wildly apart.

If you had a region of the universe that had a little bit more stuff in it than

average next to a spot that had a little bit less stuff than average, given

enough time, they would have smoothed each other out. But the inflation of the

universe ripped them apart and they became frozen in place.

Tiny fluctuations at the moment of the universe's creation unlock the genesis

of the largest structure in the cosmos.

What is the invisible force that molds trillions of stars into the Great Wall's

distinctive shape?

And what is the mysterious ingredient that glues together this cosmic mega

structure?

The Boss Great Wall is the most massive structure found so far in the universe.

Astronomers can see 830 of its brightest galaxies.

They estimate that thousands more lurk inside its vast expanse.

The wall is not a shapeless mass.

Astronomers 6 billion light years away on Earth.

See that the wall's galaxies are arranged into a defined shape.

It looks from one angle, like a fishing hook.

The structure is flat and narrow.

It consists of a mixture of stretched, tubular sections, joined together by

small and squat sections.

Why is the Great Wall this distinctive shape?

Six billion light years is too far away for telescopes to see in detail how the

Great Wall joins together.

But clues exist closer to home that can help astronomers unlock the mystery.

Despite its ridiculous size and forbidding distance, the Great Wall is

made of the same things we see around us. We can learn about that and then

extrapolate that to understand how the Great Wall itself behaves.

We live in the Milky Way galaxy.

It is a giant disc-shaped spiral that measures 100,000 light years across.

The Milky Way is just one of a cluster of over 30 closely spaced galaxies known

as the Local Group.

And the Local Group is just a single fragment of an even greater gathering.

a collection of hundreds of galaxy clusters.

Astronomers call this type of structure a supercluster.

The Milky Way is part of a supercluster that is 100 million light-years wide.

And this megastructure is not unique.

Superclusters pop up everywhere that astronomers look.

Almost every galaxy is part of a supercluster, and there are millions of

these superclusters in the universe.

Each one of them has tens to hundreds of thousands of galaxies in them, and each

one of those galaxies has billions of stars.

The supercluster is the key that unlocks why the Great Wall is its distinctive

shape.

The Great Wall measures 1 billion light years across.

Breaking it apart reveals that it is a cluster of four superclusters.

Two of them are large, densely packed tubes, each more than 500 million light

years long.

And two are smaller superclusters, each over a thousand times the mass of the

Milky Way.

These superclusters join together to form a true heavyweight champion of the

cosmos.

The largest single object we have ever seen.

Astronomers now know that 9 out of 10 galaxies sit inside a supercluster.

What brings galaxies together into these vast structures?

What mysterious force sculpts something as large as the Great Wall?

A clue lies in the way that everything moves through the universe.

Objects attract each other.

The more massive the object, the greater the pull.

Scientists call this force gravity.

Gravity is the invisible glue that connects everything.

If the universe is a symphony, then gravity is its great conductor.

It choreographs the dance of everything in the cosmos. It brings clouds of gas

together to form stars, stars together to form galaxies, and galaxies together

to form clusters, including the Great Wall.

It sounds simple.

Gravity pulls objects together into ever larger structures.

But there is a problem with this idea.

When scientists use supercomputers to test this theory, they hit a snag.

From the Big Bang to the galaxies that we see today,

computers can simulate how the cosmos evolved.

When scientists simulate the formation of superclusters, these megastructures

end up loose.

as gravity isn't strong enough.

Only when they boost the gravity beyond the expected level does matter assemble

fast enough to form the superclusters that we see today.

So how can objects like the Great Wall form if gravity is too weak?

The only force in the universe that we know that brings things together is

gravity. When it comes to structures as large as the Great Wall, there just

isn't enough gravity in the galaxies together to form something that huge.

Something else must be there underlying all of it, creating more gravity than we

can account for.

Where is the missing gravity that the universe needs to make the Great Wall?

Scientists believe that a clue lies in the space around galaxies.

Gravity does more than pull on matter.

It also bends light.

When powerful telescopes photograph dense galaxy clusters, the objects

around them look distorted.

This warping of space-time by a giant cluster of galaxies can act like an

optical lens that magnifies the light of galaxies beyond it. These strange

smeared-out images all over the sky.

These are real.

This is actually what you would see if your eyes were sensitive enough. You

would see this in the night sky.

You're actually looking at the effect of gravity.

But sometimes something weird happens.

The light that comes from behind galaxies bends far more than astronomers

expect.

There must be an invisible material with huge mass and gravity around these

galaxies.

This material could unlock where the extra gravity comes from to build mega

structures like the Great Wall Scientists call it dark matter and

they estimate that it outweighs regular matter by a factor of five to one

Dark matter permeates the universe.

We can't see it, but we know it's there through its effects.

It has gravity. And if it has gravity, it must have mass.

If it has mass, it must be made of matter that we can't see, hence the

name. The question is, what is it?

The fact that dark matter appears to have weight is evidence that it is some

kind of particle.

If dark matter is made of particles, then physicists should be able to detect

it.

Today, teams around the world race to find this strange matter.

Physicists calculate that in theory, billions of dark matter particles hit

the Earth every second.

Powerful solar radiation showers the surface of our planet.

This drowns out the signs of the elusive dark matter above ground.

But solid rock blocks all the radiation from the Sun.

So investigators build dark matter detectors deep underground.

This is the largest of them.

It's called the LZ experiment.

It sits in an abandoned gold mine one mile beneath South Dakota.

The strange properties of dark matter mean that it is one of the few types of

particle that can pass through solid rock to reach this tank.

Inside the tank are 10 tons of liquid xenon.

This dark matter experiment uses xenon because it's really stable. It's not

radioactive. It doesn't undergo chemical reactions.

So if something happens in that xenon tank, it has to be external.

The theory is that certain types of dark matter will hit a xenon atom and create

a characteristic flash of light.

And if we see one of those, we have our dark matter smoking gun.

The discovery of dark matter will finally prove what creates the extra

gravity that shapes the universe.

It could also help explain the evolution of cosmic megastructures, like

superclusters and the Great Wall.

In the early universe, it is possible that dark matter condenses and begins to

flow to form a gigantic irregular matrix.

Its massive gravity draws in the first atoms of newly forming matter in the

cosmos.

More and more matter condenses onto this scaffold and its gravitational pull

gets stronger.

So this is where stars and galaxies form.

They cling to their invisible scaffold of dark matter in huge structures like

the Great Wall's superclusters.

Dark matter is needed to give structure or scaffolding to something like the

Great Wall.

Dark matter is the critical missing ingredient for our understanding of how

the universe actually works and how structure grows and evolves within it.

Dark matter solves the riddle of how the Great Wall gets its distinctive shape.

But what lies beyond?

Is the Great Wall a once-in-a-universe structure?

Or is this vast gathering of stars just one brick in an even bigger cosmic wall?

Astronomers discover that this Great Wall is not alone.

They find similar cosmic megastructures lurking in the depths of space.

Like this, the Sloan Great Wall.

Its five stretched-out superclusters contain an estimated 6,000 galaxies.

This wall is huge.

But it is only around half as massive as the Boss Great Wall.

We're calling this the Great Wall, but it's really the biggest Great Wall. We

know of about a dozen others, and these are immense structures.

What is the link between these cosmic megastructures?

And what joins them together?

astronomers race to find the connections between great walls.

But seeing these links is a big challenge.

Matter produces light in places where it is dense enough to form stars and

galaxies.

But precisely what lies in the dark spaces between these bright spots is a

mystery.

So little of the universe is actually bright, glowing with starlight, and yet

that's really the only thing we can see in the distant reaches of space.

So we see these galaxies beginning to line up, but we don't really see the

underlying structure.

They're kind of like little fairy lights on a string, but we can't actually

connect all the little bright dots yet.

In 2019, the Very Large Telescope in Chile makes a vital discovery.

It captures an image of a cluster of young galaxies 12 billion light years

from Earth.

The young galaxy's newborn stars burn super bright.

Their intense light illuminates the darkness.

It reveals areas of space that are normally impossible to see.

This cosmic flashlight shows that immense clouds of hydrogen fill the

space between galaxies.

Discovering this material was an absolute game changer because it allows

us to see structure in the universe on galactic and intergalactic scales for

the very first time.

Astronomers think that vast invisible clouds of hydrogen like these run

between all galaxies.

But how do gas and dark matter weave together megastructures like great walls

that are separated by billions of light years?

A clue lies in the way gravity shapes the superclusters that make up these

great walls.

When you think about things pulling together under the force of gravity, you

would assume the universe is very good at making big blobs, everything just

orbiting around everything else.

But at the scale of superclusters, the galaxies form very interesting, very

well-defined shapes.

The observable universe contains an estimated 10 million superclusters.

The galaxies in 70% of these structures appear to line up.

They form into long stretched out shapes called filaments.

Astronomers believe that a quirk of gravity unlocks why so many are this

shape.

The Big Bang scatters energy across the universe, where it condenses into

huge elongated clouds of matter.

Gravity squashes an elongated cloud to a point where dense areas collapse to form

stars and galaxies.

Gravity attracts the densest parts of the cloud to each other.

The gas and galaxies flatten into a sheet.

Gravity then draws together the densest parts of this sheet.

to eventually form long filaments.

Astronomers think that the way gravity molds gas and dark matter on a huge

scale provides a clue about how the Great Wall joins to the wider universe.

Starting soon after the Big Bang, gravity sculpts the gas and galaxies of

the early universe into sparkling tube-like superclusters.

The bright galaxies inside four superclusters form the distinctive shape

of the Great Wall.

But the Great Wall may not float alone through space.

Its superclusters could stretch for many more millions of light years and

intersect with similar tubes of gas to make a cosmic web, of which the Great

Wall is merely a part.

Galaxies are not randomly strewn throughout the universe.

They form into superclusters.

Superclusters aren't randomly strewn throughout the universe either. They

form a pattern, a web of filaments.

This pattern is telling us about how the universe is built on an immense scale.

The superclusters in the Great Wall could be just the bright, visible

sections of super long, invisible threads of gas and dark matter.

that crisscross the universe.

Where precisely do these invisible filaments lie?

What is the shape of this cosmic web?

Researchers at the University of California Santa Cruz believe that one

of the Earth's strangest creatures might unlock the mystery.

This is a slime mold The entire organism consists of a single

giant cell Slime molds are really exotic.

They're not an animal nor a fungus nor a plant But they're found ubiquitously

across the planet in the damp dark corners of the world Slime molds thrive

on bacteria and rotting matter

But this food is spread across a wide area.

So this strange organism has a cunning strategy to scavenge the greatest area

using the least amount of energy.

It grows a feeding network.

A slime mold is a very simple organism. It lives in decaying vegetation.

They actually send out little tendrils. The tendrils connect and the slime mold

grows over time.

So you have this larger web-like structure evolve.

The researchers believe that the algorithm the slime mold uses to search

for food could predict how the universe weaves great walls into a cosmic web.

The researchers decode the slime mold's special algorithm and inject it

into a 3D model of 37,000 nearby galaxies.

This is the result.

The virtual slime mold sees the galaxies as food and builds a network between

them.

This network predicts the exact paths of the invisible filaments of gas and dark

matter that weave the universe together.

But does the slime mold match the pattern of the real cosmos?

There is a way to test the simulation.

The Hubble Space Telescope photographs nearly a thousand quasars between 1990

and 2020.

Astronomers think that quasars are supermassive black holes at the hearts

of distant galaxies.

The massive gravity of quasars accelerates and superheats the matter

that falls into them.

This energy produces an intensely powerful light that is perfect for

checking the slime mold's predictions.

Some of the predicted gas filaments sit directly in front of a quasar.

The pictures from Hubble show that the light it sees coming from these quasars

is dimmer than expected.

It is strong evidence that invisible gas filaments block some of the light.

The cosmic web that links superclusters and great walls across the universe

really could be the shape of a slime mold.

The universe really produces some weird connections, and one of the weirdest is

the fact that the networks and connections that the slime molds make

with each other looks a lot like what we see in a large-scale structure.

Scientists do not know for sure why the slime mold and the universe grow into

similar shapes.

Both networks appear to follow a hidden mathematical rule.

that allows them to fill the maximum volume of space with a limited amount of

matter.

Astronomers believe that great walls are the largest, densest patches of galaxies

and gas in a vast cosmic web.

Thanks to the tiny slime mold, we can now see exactly how these vast

megastructures connect across the universe.

How big can the mega structures get and what does the future hold for the Great

Wall?

Could there be something even bigger than the Great Wall out there waiting to

be discovered?

Astronomers think that the answer hinges on the outcome of a battle between two

forces that shape our universe

The first force is the pull of gravity Gravity attracts things together and not

just that the closer you are to something the stronger the gravitational

force So all across space you're attracted to your nearest neighbor that

builds up structure from galaxies to clusters to super clusters and larger

The second force is the push of the Big Bang

On the very largest scales, we have the force of expansion left over from the

Big Bang itself that has caused the universe to grow 93 billion light years

across over the last 14 billion years.

The universe will build ever more massive structures if gravity beats the

force of expansion.

But structures can't grow bigger if expansion pushes matter out of reach

faster. then gravity can gather it together.

Finding a structure greater than the Great Wall rests on unlocking which of

these two factors is the more powerful.

And so, to figure out how much pulling together and pushing apart there are, we

need to make a very precise measurement of the expansion rate of the universe.

Astronomers at Arizona's Kitt Peak Observatory get a front row seat at this

cosmic contest.

This is the Mayol telescope.

It measures the distances between thousands of galaxies.

Astronomers calculate the average distance between galaxies today.

They then compare these with the distances between galaxies in younger

parts of the universe.

This comparison charts the speed of the expansion.

The data from the Mayall telescope reveals something extraordinary.

The expansion is getting faster and faster over time.

This acceleration puts ever greater distances between the matter the

Universe needs to build superclusters of galaxies.

This means that there is a limit to how big Great Walls can get.

It is possible that the Boss Great Wall is the largest structure in the

Universe.

Why is the expansion of the Universe accelerating?

And what does it mean for the future of the Boss Great Wall?

A clue lies in what happens in the coldest, darkest and emptiest parts of

space.

In 2019, scientists in Zurich, Switzerland, detect tiny energy

fluctuations in a vacuum chamber, chilled to just above absolute zero.

These bursts could come from strange, exotic particles that pop in and out of

existence.

Their energy produces a tiny outward force, like a small push on the fabric

of space itself.

In places where there is lots of matter, like around planets, stars and galaxies,

these minute effects go unnoticed.

But it could be a different story in parts of the universe that are almost

completely empty.

Like the giant voids in between the threads of the cosmic web.

This whole time, we've been letting our eyes be drawn to the bright parts of the

universe, this beautiful cosmic web that stretches across space.

But if you look in between the filaments, there are these vast voids

where there is almost nothing.

And could it be that very empty regions of space behave in some fundamentally

different way where there really is nothing?

Scientists pin this different behavior on a weird hidden force called dark

energy.

The way that dark energy works could explain why the expansion of the

universe is accelerating.

The tiny push from dark energy expands the space around it like an oil bubble

that grows in clear water This new

space contains dark energy The push from this new dark energy creates

even more new space This never-ending cycle continuously increases

the amount of dark energy

The expansion of the universe accelerates.

And astronomers think that this spells doom for the Great Wall.

As the megavoids grow, they flood the space around them with dark energy.

And the Great Wall is right in the firing line.

The dark energy rips the superclusters from the Great Wall.

and tears them apart.

The dark force is so strong that it shreds superclusters into their building

blocks.

Eventually, the galaxies themselves disintegrate.

They leave stars and planets drifting alone through space,

with skies devoid of starlight.

We live in a special time in the universe, the era of superclusters. The

universe is expanding.

Moreover, we think that that expansion is accelerating.

It's expanding faster and faster every day.

This spells doom for the Great Wall, because eventually dark energy will tear

clusters apart.

The Great Wall is a wonder of the cosmos.

It is a vast assembly of gas and galaxies that stretches over a billion

light years of space.

Even though the Great Wall is one of the largest known structures in the

universe, it's so ephemeral and so easy to miss. And so its actual discovery has

been one of the great achievements of observational astronomy.

Astronomers now know that the Great Wall is just one small part of a network that

holds everything in the universe together.

A giant cosmic web of trillions of galaxies stretching billions of light

years across the universe filling the entire space.

It's just mind-blowing.

Quantum fluctuations frozen in time trillions of a second after the Big

Bang. lay the foundations of the Great Wall.

Dark matter then chisels out the megastructure when the universe is

young.

Dark energy threatens to tear it down billions of years from now.

The universe appears to be almost ripping itself apart.

Even superclusters, even clusters of galaxies may not survive dark energy.

We have never seen anything quite like the Great Wall.

and we may never see anything quite like it in the future.

But while it lasts, it is one of the biggest highlights of the cosmos.