All language subtitles for How the Universe Works (2022) - S10E03 - Hunt for Dark Matter_track3_[eng]

[narrator] Scientists believ
there is a hidden substance
deep in space

that keeps the cosmos runnin

But is that substance real?

We've never seen dark matter,
it's completely invisible,

but we know
that it has to be there.

Not only can you not see it,
you couldn't really touch it
or taste it, or smell it,

and yet it is all around us
it affects everything
that we do.

[narrator] After searching
for decades,

we still don't understand
this inexplicable substance

[de Rham] We know dark matte
is there because we feel
its strong gravitational pul

but it just doesn't
want to talk to us.

[narrator] There's evidence
that dark matter

makes up 85%
of all the matter
in the universe.

We can see dark matter
holding galaxies together

and ripping
other structures apart,
we even see it bending light.

Dark matter itself has been
around since the beginning
of the universe.

Without dark matter,
we wouldn't be here.

[narrator] But if you can't
see dark matter

and if you can't touch it,
does it really exist?

[narrator] The Hyades
star cluster.

This family of 700 stars
is 150 light years from Eart

At the scale of the universe
it's in our backyard.

Hyades is actually
close enough to Earth

that you could see it
with your naked eye.

When you look up
at the night sky,

Hyades is in that V-shape
in Taurus the Bull.

[narrator] For most
of its 650-million-year
lifetime,

the Hyades enjoyed
a peaceful existence.

But something
is breaking the calm.

The Hyades cluster
is one of the most
well-studied clusters of stars

we have in the entire sky
and yet there's something

very deeply mysterious
going on.

[narrator] Two star tails
extend from the cluster
center,

they should be roughly equal
but one tail
is hemorrhaging stars.

Something is disrupting it,
there's something exerting
a force on it

that's ripping stars
out of their orbits.

[narrator] Something
with immense
gravitational pull

has passed by the cluster

and robbed it of stars.

In order to be gravitationally
pulling stars out
of an object like Hyades,

you need to have an incredibly
massive structure,

as much as 10 million times
the mass of the sun.

[narrator] This monstrous
cosmic mugger
should still be visible,

but when we point
our telescopes

to where it should be,
that region is empty.

There's nothing there
and I mean nothing.

And not a little bit
or something dark,
or something small,

but there's literally
nothing that we can see.

[narrator] We know somethin
is out there,
invisible and powerful.

And whenever we witness
these unseen assaults,

a prime suspect
gets called in,

a phantom of physics,

dark matter.

So what can we confidently s
about this mysterious
cosmic substance?

It does not emit light,
it does not reflect light,
it does not absorb light.

The only thing we know
about dark matter
is that it has gravity.

We're not even really sure
it's matter at all.

It's just that that's the only
thing we know,
that it has gravity.

[narrator] We may not be abl
to see or touch dark matter

but we are very good
at finding its fingerprints
all over the universe.

We can see dark matter's
use of gravity to break
and bind structures

and we've been spotting
its handiwork for decades.

Let's rewind back to 1933.

Swiss-American physicist,
Fritz Zwicky tracks
strange movements

in a far off collection
of galaxies called
the Coma cluster.

He knows he's not
seeing the whole picture.

Some galaxies are speeding
around the cluster
at inexplicably fast rates.

[Bullock] Zwicky is looking
at these galaxies

and if the only mass
that was there were the other
galaxies you can see,

you would expect
these galaxies to be moving
at about 50 miles a second,

then they would stay bound
to each other
and not fly apart.

Instead, he sees them moving
at 1,000 miles a second.

[narrator]
At these velocities,

galaxies should be flying of
the cluster like sparks
from fireworks.

Zwicky realized there had
to be extra stuff,

in his words, Dunkle Materie

Dark matter.

Dark matter.

Dark matter.

[narrator] It becomes clear
that Zwicky's Coma cluster
isn't an isolated case.

Astronomers begin seeing
the same dynamics
within galaxies themselves.

In systems governed
exclusively by gravity,

objects farthest away
from the center

would take the longest
to complete an orbit.

But in many galaxies,
stars on the outside
are orbiting

at almost the same rate
as those in the core.

[Thaller] It's almost like
a photograph record.

Every part of that record
spins around
like a solid disc.

The stars are going too fast
to stay bound to the gravity
of the galaxy.

They should just
fly right off into space.

[narrator] Physicists come u
with an explanation.

Galaxies sit in a giant hal
or ball of invisible
dark matter.

And it's that extra mass
that allows the stars
to turn fast

all the way out
to the galactic rim.

[Thaller] Think about actual
taking a disc of dough

and spinning it
to make a pizza.

The more you spin it,
the more those outer regions
go farther and farther away.

Eventually, the dough
just goes flying everywhere,

that's what would happen
to a galaxy if it weren't
for dark matter.

Uh, as you spin pizza dough
and you spin it faster
and faster,

it does hold itself together

because there's all this yummy
gluten that's acting
as a glue.

Dark matter is the gluten
of our universe.

[narrator] By calculating
the mass needed to bind

those speeding outer stars
to the galaxy,

physicists are able
to estimate

how much visible matter
there is compared
to dark matter.

The results are staggering.

[Tegmark] All the stuff
we thought existed was just
maybe 15% of our universe.

That's like if you go
to a restaurant

and leave like the measly
15% tip, you know,
that's what we are.

I mean, not even
the majority substance.

[narrator] We may not
be able to see it,

but dark matter makes up
some 85% of all matter.

Wherever we look, we can se
its gravity having effects.

It glues galaxies
like our Milky Way together

And a close look
reveals dark matter
can also bend light itself.

It's called
gravitational lensing.

A massive object
can bend space and time,

and light must follow
the curves of that space
and time.

[narrator] Gigantic clumps
of any matter create
a gravitational lens.

Dark matter showed
its space-warping power
in a trick it played

with a gigantic explosion
in a far off galaxy cluster

Supernova Refsdal was first
detected in November of 2014

Supernova Refsdal
is actually one of my favorite
recent results

in all of the astronomical
literature.

That result blew me away.

So a star explodes,
light is emitted
in all directions,

and some of it makes its way
towards the Earth.

So far so good.
This is very standard.

So the flash appears

and then,
another flash appears.

We see it again,
and again, and again.

[Mingarelli] We see
the explosion go off
in four parts of the sky.

And then, a year later,
a fifth explosion goes off

in a totally different part
of the sky.

What's going on?

[narrator] Analysis proves
that these multiple explosio
are the same supernova.

-[explosion]
-But between this one
dying star and our telescope

sits a giant mass
of dark matter,

a huge gravitational lens.

What that means is that some
of these rays of light
will take much longer,

more complicated paths
through this region
of space time.

[narrator] The dark matter
lens turns one supernova

into a fireworks display
lasting an entire year.

Dark matter affected
the trajectory of light
from this supernova so much

that for some
of those trajectories,

it added a whole light year,

it took a whole extra year
for light to reach us.

[narrator] Something is ver
definitely out there

distorting our view
of the cosmos.

It's a potent clue
that dark matter is real.

Now, new evidence suggests
that without it,
we might not exist at all.

[narrator] The cosmos
is filled with an unseen
substance,

its mass even bends starligh

Gravitational lensing sugges
dark matter holds our entir
universe together.

For decades, this specter
of space has haunted us.

We've never been able
to pin it down.

In 2021, an international te
ran a virtual experiment
to try to predict

where dark matter should
be by letting computers

map out
where we think it lives.

[Bullock] Because we think
we know how it behaves,

we can model what it should
be doing in supercomputer
simulations.

[narrator] The team taught
the computer how dark matte
bends light,

then applied computational
power to 17,000
unexplored galaxies.

The model created
a dark matter map.

I think a lot of people,
when they imagine the universe
on the larger scales,

think it's sort of boring,
everything's uniform.

But that's not what we see.

What's amazing is that
on the larger scales
of the universe,

we see a very
particular pattern.

When we zoom out,
we see this magnificent
structure,

this cosmic web
that's created by dark matter.

[narrator]
The interweaving tendrils
of dark matter stretch

for thousands of light year
across the cosmos.

At the junctions where matte
is concentrated,
we find galaxies form,

illuminating
the dark scaffold.

If dark matter exists,
scientists believe

it makes up 85% of the matte
in the universe,

and also controls
the remaining
15% regular matter,

like stars, planets, us.

If they're right, dark matte
played a critical role

in actually building
the universe we see today.

2021.

Astronomers using
the SkyMapper observatory
in Australia

trains specialist optics
on a dwarf galaxy
called Tucana II.

The SkyMapper's filters
split up the starlight
into a spectrum of wavelengt

revealing some
very ancient light.

[Tremblay] One of the best
clocks that we can put
on the universe

is the progress of chemistry.

Right?

The build-up of more complex
elements over time.

Stars are nothing
if not factories
of chemical complexity.

They slam, uh, particles
together and create
heavier elements,

right, through a process
called fusion.

The later the generation
of star, the more chemicall
complex it is.

[narrator] Tucana II's
spectral signature
reveals its stars contain

very few of these heavy
complex elements.

A clue that lets
astrophysicists calculate
the age of the galaxy.

These are very, very old stars
from the very early days
of the universe

when the gas
in the universe was not
that chemically complex.

Tucana II might be one
of the oldest known structur

that we can see
in our local universe.

It could be as old
as 13 billion years.

You know, almost as old
as the universe itself.

[narrator] This grand old la
of a galaxy is a tiny thing

Barely 3,000 stars.

And yet, way out
on her galactic rim,

stars hurdle around
at breakneck speed.

[Plait] When you look
at the mass of this
ultra-faint dwarf galaxy,

it only has a few thousand
times the mass of the sun.

That's really small.

And at the speed it's moving,
it should fly apart.

[narrator] Tucana II doesn'
break up because
it's glued together,

apparently by an incredible
amount of dark matter.

When you look at a galaxy
like our Milky Way,

it's about 85% dark matter,
which is a lot.

But with Tucana II,
it's more like 99%.

[narrator] Tucana II is old
among the oldest galaxies
in the universe

and it is packed full
of dark matter.

Simulations suggest
this dark matter played a ke
role in shaping Tucana II

and other very early galaxie
right from the beginning,

gathering regular matter
into clumps and building
the first galaxies.

[Thaller] The importance
of dark matter
really can't be overstated.

It has actually controlled
the way matter has evolved

since the beginning
of the universe.

It brings matter together.

You need this underlying
structure of dark matter
to make it all happen.

[narrator] Scientists think
that for billions of years
as the early universe grew,

dark matter called the shots

Without its gravity,
structures like the Milky Wa
wouldn't have formed.

We've seen dark matter's
light-bending effects.

We've even deduced
where it should be.

Dark matter really does
appear to exist,

but this evidence
is indirect, circumstantial

To get conclusive proof
that dark matter exists,

don't we need to find some?

If we could find a lump
of dark matter [laughs],

um, that would be one
of the greatest discoveries
in all of nature,

in all of our history, right?

Because we would understand

one of the most fundamental
components for how
our universe works.

Dropping the title,
they love that.

[narrator] It's time to hun
for dark matter itself.

Could it be hiding
in the darkest place of all

Black holes.

[narrator] Scientists believ
an invisible substance

is pulling the strings
in our universe.

But until we see it, sense i
perhaps even touch it,

dark matter is just a theory

Sometimes though,
ideas dreamed up

by scientists come true

like black holes.

Once the stuff
of science fiction
and children's nightmares,

black holes today
are confirmed reality.

So black holes
and dark matter have a ton
of similarities, right?

You know,
an unseen collection of matter

that creates an enormous
gravitational field, check.

It bends light
and causes gravitational
lensing, check.

Tests the boundaries
of known physics, check.

[narrator] It seems crazy
to even ask,

but could our search
for dark matter

end in an idea more
than 100 years old?

Could dark matter
be black holes?

Black holes appear
when stars explode.

And their remaining mass
crunches down into a sphere

so dense even light
can't escape its gravity.

But that's where
the black hole dark matter
theory stumbles.

We know
that black holes happen.
We know how they form.

And we also know that there's
nowhere near enough of them
to be dark matter.

[narrator] Not enough stars
have lived and died
in the history of the univer

to create 85%
of the matter in it.

If dark matter is made up
of black holes,

they would have
to be an entirely new type.

It's possible that these black
holes are of a type
that we've never seen before.

They could be
primordial black holes.

[Hopkins] Primordial black
holes are an idea.

A theoretical concept
at this point

that we've never seen,
but they could exist.

If primordial black holes
are real then the universe
is flooded with black holes.

[Hopkins] The smallest coul
have the mass of Mount Evere

packed into the size
of one atom.

The biggest could be hundreds
of thousands or millions
of times the mass of the sun.

[narrator] Stephen Hawking
first suggested
that primordial black holes

could be dark matter back
in the 1970s.

The idea centers
on what happened
during that intangible momen

13.8 billion years ago,
the big-bang.

Theory says
that primordial black holes
formed in the first fraction

of a second
of the early universe.

It's that time
between when the universe

goes from a pinprick
to this giant inflating
ball of gas.

[narrator] In these first
moments of the universe's
existence,

matter is packed
incredibly tightly.

But it's not quite
evenly spread.

Even the tiniest fluctuation
in density could trigger
gravitational collapses.

In other words,
black holes would
be forming everywhere,

theoretically,
in huge numbers.

By the time one second
has passed in our universe,

you're already making
black holes

thousands, hundreds
of thousands of times
more massive than our sun.

[narrator] The collective ma
of these objects could be va

but could they be 85%
of the universe's matter?

[Sutter] If primordial
black holes really do exist

there might be enough
to explain the dark matter.

[narrator] It's a tantalizin
possibility, but there's on
pretty big problem.

For most scientists,
the physics
of the very early universe

is incomplete
and hard to trust.

Generations of physicists
dismissed primordial
black holes

as myths, fantasies,
astrophysical unicorns,

until that is, an earthshaki
crash in space.

May, 2019.

A violent cosmic
event rocks the USA.

How violent?

Well, the physical distance
between Louisiana
and Washington state

is stretched by nearly
the width of an atom which
is bigger than it sounds.

The Laser Interferometer
Gravitational-Wave Observato

detects this wobble
in space time.

This is the biggest
gravitational wave event
that LIGO has seen.

[narrator] This cosmic
disturbance seems to come
from colliding black holes

but crucially not the ordina
dead star type.

In this LIGO detection,
one of the black holes
is 85 solar masses.

There's no way that a star
could've made that black hole.

[narrator] Physicists believ
there's a range of masses

where dying stars
can't collapse
into black holes.

Instead, stars in this zone
become insanely hot
and rip themselves apart

leaving nothing to crunch do
into a black hole.

Eighty-five solar masses
sits right in the middle

of this so called forbidden
mass range.

The black hole
that LIGO detected
can't be a dead star,

but in theory
it could be primordial.

Could this discarded theory
of dark matter
be back in fashion?

The LIGO detections come up
and everyone says,

"Oh, right,
primordial black holes.

Maybe we should pay
more attention to that."

Primordial black holes can
be really appealing

because they would solve
the dark matter problem.

But unfortunately,
it's not that simple.

The thing with flooding
the universe with primordial
black holes

is that you expect
a lot of collisions.

And so LIGO shouldn't
have seen one,

it should have seen a thousand
of these collisions
and we don't.

[narrator] Many scientists
doubt what LIGO saw
was a primordial black hole

To them, these beasts remai
fairytales of physics,

red herrings in the quest
for solid evidence
of dark matter.

Does dark matter exist?

Or are we chasing shadows?

Some scientists think
it's not only real,

but the dark matter
is within our grasp,

and that it's flying
through our bodies right now

[narrator] We think 85%
of the universe's matter
is dark.

And yet, we've never found
a speck of it.

We can't prove
dark matter exists.

Regular matter is made up
of everyday particles,
like electrons and protons.

Scientists wonder
if dark matter is also
a type of particle.

One of the leading candidates
for dark matter

are these things called
Weakly Interacting
Massive Particles.

They're massive particles
like protons and electrons
and things like that.

But they don't interact well
with normal matters,
so they're weakly interacting.

And they just have this name
because it's awesome
to call them WIMPs.

[narrator] For decades,
scientists have struggled

to find these shy
theoretical particles.

[Oluseyi] The very first
physics research I ever did
in my life

was about actually
measuring directly
dark matter particles,

these so called WIMPs.

And if they exist,
then there will be a flux
of millions of them

through my hand right now,
just by holding out
right here.

If dark matter is actually
made of WIMPs,
if these particles exist,

then we're actually living
basically in a sea of them.

It surrounds
and penetrates us and it bin
the galaxy together.

[narrator] WIMPs don't play
by our rules.

They barely interact
with the world
of regular matter,

so they're hard to detect.

But when they play
with each other,

sparks fly, intense flashes
that we just might
be able to see.

[Plait] As the theory goes,
WIMPs will self-annihilate.

WIMP A and WIMP B
get too close together, poof,

they explode
and they create gamma rays.

[narrator] Gamma rays
are high energy light,
making them easy to spot.

Scientists point
their detectors at the cente
of the Milky Way,

where they believe the WIMP
collision rate should
be especially high.

[Plait] We have a 4 million
solar mass black hole there

There are billions
of stars there.

That's where most of the mass
of the galaxies is densest.

So any WIMPs orbiting
the galaxy will feel
this natural attraction

towards the center
and fall toward it.

[narrator] The Fermi Large
Area Telescope scoured

the center of our galaxy
for more than 10 years.

It detected lots
of gamma rays,

but scientists couldn't tel
if they came
from colliding WIMPs.

The Galactic Center is a mess.
It's like downtown
of a city, right?

That's where everything is,
where all the hustle
and bustle is.

There are stars
exploding there,

just tons of stars, gas,
magnetic fields, a black hole,

a lot of sources
of gamma rays,

so it's very difficult
to tease out the signal.

[narrator] Downtown Milky Wa
was a washout.

So the scientists turned
their attention to planets

living in less noisy
ZIP codes,

where WIMP collisions
should be easier to spot.

One place where you might see
evidence for WIMP collisions

is actually the cores
of exoplanets.

Turns out exoplanets might
be the best dark matter
detector we have.

You can use giant planets
orbiting distant stars

as laboratories
to understand dark matter.

[narrator] We know gravity
should attract WIMPs.

The more gravity,
the more dark matter particl
come together.

Scientists suggest that WIMP
congregate inside the cores

of the Milky Way's
largest gas planets.

In these super-sized
gas giants,

WIMPs could collide,
annihilate, and release
gamma rays.

If there are these WIMPs
that are collecting
the centers

of mass of exoplanets,
the annihilation
of that dark matter

can heat those exoplanets up.

If you have a WIMP-heated
exoplanet, and that's just fun
to say,

this thing is going
to be warm,

it's gonna be warmer
than the heat of space,
which is very cold.

So what you need
is an infrared telescope,

something that sees
an infrared light
and is sensitive enough

to be able to measure
the temperatures
of these things.

[narrator] But a dedicated
telescope like this
won't launch until 2028.

For some dark matter hunters
that's too long to wait.

They argue that WIMPs
do have one characteristic

that should allow us
to detect them
right here on Earth.

The key to detecting WIMPs
is in their name,
it's the W-I.

They're weakly interacting.
They're not not interacting.

They do interact,
it's just very weak
with matter.

And that means that
there are the rare occasions

where it will smack
into a particle
of normal matter

and then there are effects
that we can observe.

[narrator] Scientists in
Gran Sasso in Central Italy
watch for a spark of energy

generated when a WIMP hits
an atom of regular matter.

Their detector, a tank
of super cooled xenon

built thousands of feet
beneath the Earth's surface

The beauty of putting
this detector under a mountain
is that you've got all of this

rock and soil
and everything else

which is blocking
a lot of background noise.

When you're looking
for a WIMP interaction,

you're looking for something
that's very rare
and something very subtle,

so you don't want
other things going on.

You don't want other particles
coming in and messing up
your experiment.

These Weakly Interacting
Massive Particles will pass
right through that mountain,

and then if they smack
into a xenon atom,
we can look at it and go,

"Ah, that was
a dark matter particle."

[narrator] Detecting a WIMP
could be definitive proof
that dark matter exists.

In 2020, the scientists
spotted something
in the results.

But was it
the elusive evidence

or a ghost among the stars?

[ambient music playing]

[narrator] Scientists believ
they can prove
dark matter is real

by detecting WIMPs.

An experiment buried
deep beneath
an Italian mountain

spotted unusual activity
in a tank of regular matter

pure liquid xenon.

[popping]

So a WIMP detector,
like the XENON1T,

waits for a little WIMP,
tiny, tiny little particle

to hit an atom
of normal matter,

and that creates a vibration.

And we can see this entire
block of xenon shake

just a little bit
from that little,
subatomic collision.

[narrator] The intensity
of the vibration

from the particle collision
is critical.

In theory, a WIMP striking
a xenon atom should generat
a powerful shock.

The vibrations XENON1T
detected were too weak.

When a WIMP comes through,
it smashes into the atom.

It seemed like here
something was just sort of
rattling the electrons

on the outside of the atom.

So whatever is causing
these detections was likely

something much smaller
than a WIMP.

[Sutter] Let's take
these results at face value

It... If they're correct,
it's telling us

that the dark matter
isn't a WIMP,

but something
much, much smaller

and something
much, much lighter.

[narrator] The results sugge
that what hit the xenon

was actually a much
smaller theoretical particl
called an axion.

[Bullock] Axions are really
weird particles,
incredibly light.

In fact, almost zero mass
is possible for an axion.

An axion is no bigger

than 150 billionth
the size of an electron.

Compared to a WIMP,
an axion is like a soccer ball
compared to our sun.

[narrator] The sheer tinines
of axions makes them seem

like an unlikely candidate
for dark matter.

If dark matter is real,
it makes up 85% of the matte
in the universe.

To account for all that mass
we would need an almost
unfathomable number of axion

142 trigintillion of them,
in fact.

That's 140
with 93 zeros after it.

If axions exist, space must
swimming with them.

They must be packed
into every corner
of the cosmos.

When regular matter
clumps together,
it forms stars.

So, to prove
dark matter exists,

maybe we should be
looking for dark stars.

There's no reason
they can't exist.

There's even a name for them

Ghost stars.

[Bullock] They're very
weird objects.

These ghost stars are like
nothing we would ever see
in the night sky.

[narrator] We've never seen
a ghost star.

They are theoretical object
made of hypothetical axions

But in theory,
ghost stars should form
like any other star,

pulled together by gravity.

They would be gigantic,
super dense objects
floating through space.

They could reach the mass
of tens of millions of suns

But because they are made
of dark matter,

ghost stars would produce
no energy and emit no light

They would be transparent
to both light and matter.

If you were right next to it,
you wouldn't
even notice it, right?

If we sent a probe through it,
it'd sail right through it

uh, and once it passed
through, it would be
pulled back by its gravity.

[narrator] 85% of the matte
in our universe could consis
of transparent orbs

made of infinitesimally
small, dark matter particles

But do these
invisible stars exist?

The evidence is thin, but..

[tape rewinding]

[narrator] Rewind back to th
LIGO detection in 2019.

The gravitational wave
detector picked up the signa

of two massive
objects colliding.

[pulsating explosion]

We call the event GW190521.

Most scientists agree this w
a black hole collision.

But could it have been
clashing ghost stars?

[Plait] If there are
ghost stars out there

and they can interact with
each other gravitationally,

they may collide.

And when they do,
they would emit
gravitational waves

and it would look
a lot like
two black holes colliding.

In fact, it would look
theoretically very much
like GW190521.

[narrator] One collision,
two explanations.

Primordial black holes
or ghost stars,

LIGO can't tell them apart.

Do these ideas bring us
closer to proving
the existence of dark matter

Or are we just hurtling
further down
a weird physics rabbit hole

[Chiara Mingarelli]
Primordial black holes,
ghost stars, axions,

this is all very
exotic physics.

We can't take for granted
that any of this is real
or that it's not real.

We just don't know.

Dark matter is irritating.

[groans] We know
it's out there.
We see its effects, right?

But we can't see
the dark matter
and that's frustrating.

And it's like a lot
of young fields in astronomy.

We have way more ideas
than we do hard observations.

[narrator] We have ideas,
we have theories,

but without direct
observations, we just can't
back them up with solid proo

The more we look,
the harder it is
to find dark matter.

Maybe it's primordial
black holes
from the early universe.

Maybe it's a sea of particle
that flow right through us
every day.

Or maybe it's gigantic,
transparent ghost stars.

Perhaps it's the combined ma
of Santa's sleigh
and the Easter Bunny's baske

Or maybe all our physics
is based on questionable mat

[narrator] 85% of the stuff
in the universe
is missing in action.

The search for this
dark matter looks hopeless.

This problem of dark matter
is really a tough one.

Everything that
we've predicted

and then gone and looked for,
we're not finding.

It's starting to become
a huge embarrassment.

Surely something
so fundamental

to our cosmology
should be detectable.

And yet, it remains elusive.

[narrator] We're stumbling
blindly around the limits
of our understanding.

As of right now, there are
zero direct observations.

Maybe dark matter
doesn't exist after all.

Instead of searching
for an invisible substance

affecting the universe
with its gravity,

maybe it's gravity
we don't quite understand.

If you're looking in a galaxy
and it's spinning
way too quickly,

either there's a new
ingredient in the galaxy,

like dark matter,
that holds it all together.

Or you're misunderstanding
the laws of physics.

[narrator] To describe
the effects of gravity,

we use the nearly 350-year-o
math of Sir Isaac Newton.

Maybe to explain
the excess gravity
we see in the universe,

it's not extra matter we nee

It's better math.

Although we understand
very well how gravity works

here on Earth
and in our Solar System,

perhaps when you get up
to galactic scales,

it actually behaves
just slightly differently.

And if that were the case,
you can kind of
tweak that idea

until it fits the data we see
of how galaxies
are spinning around

without needing dark matter.

[narrator] Questioning
the math of a legend

of physics might sound
like sacrilege,

but to solve
the dark matter conundrum,
it has been done.

It's called Modified
Newtonian Dynamics
or MOND.

Modeling galaxies
with this math produces
very different results.

On its surface,
MOND is not a bad idea.

In the same way that we would
normally program a computer

to include dark matter
in our simulations,

you can take that out,
and instead program it

with a different law
of gravity with MOND.

And then, you can set up
a kind of spinning mass of gas

and it does seem
to be possible with MOND

to get things settled down
and look a bit
like a real galaxy.

[narrator] Changing the law
of gravity
accurately recreates

the super-fast spin
astronomers see
through their telescopes.

No need for dark matter.
It doesn't exist.

Case closed?

Not by a long shot.

With anything bigger
than a galaxy,

this artificial physics
breaks down.

MOND does really well
on galaxy scales,

but when you zoom out
and you go to larger

and larger structures
in our universe,

like clusters of galaxies,
big, big structure,

you see that MOND by itself
can't reproduce
all of our observations.

There's something missing.

Dark matter.

Dark matter,
dark matter, dark matter.

Dark matter.

In MOND, you still have
to invoke the existence
of material you can't see.

[Andrew Pontzen]
It basically introduces

some of its own
dark matter as well,

which kind of negates
the point of having MOND
in the first place.

[narrator] MOND doesn't
replace dark matter.

The universe still needs
something to hold it togethe

We just don't know what it i

But there are plenty
of new ideas flying around.

In my theory, the dark matter
is a super fluid.

[narrator] It's a radical
new theory of dark matter,

particles not
acting individually,

but flowing as one invisibl
mass around the galaxies.

A super fluid is like
an ordinary fluid that flows,

but in this case,
it flows without
any resistance or viscosity.

If I pour honey,
it will flow very slowly.

It has high viscosity.

A super fluid will just flo
and never stop flowing.

[narrator] As the super flui
dark matter flows
around the universe,

eddies and waves form
large enough to engulf
entire galaxies.

The gravity of the fluid
holds the stars together.

But like most theories
on dark matter,
there's no direct evidence.

If these waves are on the size
of galaxies,

then we have to find
detectors that can detect
those types of huge waves.

They don't exist
at the moment.

[narrator] Which brings us
back to square one.

We just can't prove
that dark matter is real.

Primordial black holes,
ghost stars, WIMPs,

a super fluid sloshing
about the cosmos,

or maybe we're just using
the wrong math.

What's your money on?

If I had to wager $20
on what dark matter is.

Hmm.

I would never place money
on what dark matter is.

I just think we have no idea.

My money is on
dark matter itself is real,

but it's not
the whole picture.

I would say
left socks in dryers.

I would say remote controls
that fall into sofa cushions
and disappear.

I would love there to be

dark matter,
ghost stars, planets,
even dark matter people.

I'm going all black.

I think no current
ideas are correct.

I think dark matter
is something that
we haven't thought of yet.

[narrator]
Does dark matter exist?

Watch the space.