[SQUEAKING]
[THUNDERCLAP]
Dark matter.
Sounds pretty dramatic, huh?
Like some sinister goo under control of the Sith Lords.
Maybe that's what dark matter is.
Although you might not think so if I told you
we only call it dark matter because it
doesn't interact with light.
Yep.
Doesn't reflect light.
Doesn't absorb light.
It doesn't interact with electromagnetic radiation,
or light, at all.
That's why we call it dark.
I know.
That's a pretty physics-y statement, not quite as sci-fi.
But I promise you, there are still so many mysteries
surrounding dark matter.
And we're going to talk about what dark matter might be,
how we know it's there, how we know what it's not,
and why we might be wrong about all of this, in the end.
So dark energy makes up 69% of the energy
density in the universe.
And 26% is dark matter.
And about 5% is baryonic matter, or ordinary matter.
Wait, what?
Only 5%?
That's crazy.
This 5%, the baryonic matter, that includes me,
you, our Earth, the solar system, all of the stars,
the Milky Way galaxy, all of the galaxies-- everything
we can see.
It's all only 5% of the energy in the universe.
So then how do we know that?
And how do we know that 26% of the rest of the stuff
in the universe is dark matter?
That's the first question I had.
If we can't see dark matter-- because it doesn't interact
with light, so how would we see it,
since we only see things light-- how
do we know it's there in the first place?
Well, how do we ever know something's there
that we can't see?
What about wind?
You can't see the air moving, but you can see its effects
on other things.
Such was the case with dark matter.
All the way back in 1933, this dude
named Fritz Zwicky was studying a cluster of galaxies
called the Coma Cluster.
See, galaxies are like mean girls
and are often found in clusters.
When galaxies orbit one another, they're
often playing this game.
The faster they're going, the stronger they
need to hold on to one another.
And what's the force holding orbiting galaxies together?
Gravity!
The faster they go, the more gravitational attraction
they need to hold them together.
And we know more gravity only comes from adding more mass.
Well, no problem.
You look at how fast the galaxies are orbiting,
and you calculate how much mass is
needed for the gravitational force to hold them together.
And voila!
You find that there is mass missing.
Zwicky saw the galaxies moving faster
than the gravity of the mass he could see
could hold them together.
Oh, no!
DIANNA COWERN: This was the first hint
that there might be something out there he couldn't see.
He called it dark matter.
If you're thinking, that's it?
I could come up with a million ways
to explain what Zwicky saw.
Well, that wasn't the end.
The next evidence for dark matter
comes from a totally different but crazy phenomenon.
Check this out.
When you're looking at a star, the light from the star
is usually coming straight at you.
But when you place a giant object,
like a galaxy, or a cluster of galaxies, in between you,
then the light going this way is bent around.
Yeah, the gravity from giant objects bends light.
It's almost as if there were a giant lens
in between you and the star.
Shockingly, we call this gravitational lensing.
So depending on how much the light is bending,
we can estimate the size of the galaxy.
Because again, more mass, more gravitational
pull on the light.
So then we look at the galaxy and we calculate all the matter
we can see, and they don't add up.
There's missing mass, again!
What do you think it might be?
Now, there's even more evidence for dark matter.
And it has to do with how stars are moving around the galaxy.
But I'll let you think through that one.
OK, I'll give you a hint.
They're moving around the galaxy faster than they should be.
[MUSIC PLAYING]
Many people have speculated dark matter is just
a bunch of MACHOs.
[CRUNCH]
No, not nachos.
MACHOs-- MAssive Compact Halo Objects.
These are things like brown dwarfs, white dwarfs, even
black holes-- things that have very low luminosity,
or light emanating from them, but are small and dense.
Well, some of the missing mass might be these MACHOs.
But cosmological models of the early universe
have shown that we shouldn't see more
than this 5% level of baryonic matter.
Which happens to be the exact number that we measure.
So then couldn't dark matter just
be a bunch of dark dust and gas?
Well, same argument.
That's all baryonic matter, as well.
It can't account for all the missing matter.
So again-- what is dark matter?
Well, the leading theory is dark matter is likely a WIMP.
Yes, we have our MACHOs and our WIMPs.
Somebody has a strength complex.
A WIMP is a Weakly Interacting Massive Particle.
I'm not just going to tell you what it is.
I'm going to leave you with a few questions.
What do you think WIMPs interact weakly with?
And why do you think they might have to be massive?
These WIMPs couldn't interact with the electromagnetic force,
or light, as far as we can tell.
So scientists have set up detectors deep underground,
hoping that a dark matter particle, a WIMP,
will bump into a particle in the detector,
although infrequently, and will get a signal.
If dark matter doesn't interact with regular matter
at all, besides through the gravitational pull that we've
seen, then we have no chance of directly detecting it.
For now, we can only hope it does.
Well, I hope this gives you a sense of what dark matter might
be, what it might not be, what it is, what it's not.
But remember, science is an ever-growing and changing field
of knowledge.
And because dark matter is so new and unknown,
we might yet prove ourselves wrong on all
of these hypotheses.
But for now, we're left with the evidence.
And it points to dark matter!
Yeah!
[MUSIC PLAYING]