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Language: en
You're watching a Nova Science Now video
podcast.
[Music]
My galaxies are purple. What color are
yours?
Mine are sky blue sky.
Here at NOVA, we do a lot of things in
the name of science.
Are you ready? Ready.
[Music]
One,
two. Just ask Nova Science Now host Neil
deGrasse Tyson.
And not all of it is done purely for
fun.
Like astronomer Doug Clow, sometimes
we're trying to explore the deepest
mysteries of the universe. And dark
matter is definitely one of those
mysteries.
When astronomers refer to dark matter,
right, we we mean something that doesn't
interact with light in any way. It
doesn't give off light. It doesn't
absorb light.
Scientists like Clow think dark matter
makes up 20ome% of our universe. But
there's something else they aren't so
sure about.
We don't know what dark matter is.
We think it's probably some sort of
particle that we haven't discovered yet,
but it might turn out to be something
different. We're not entirely sure.
So why is Cloud convinced that it
actually exists?
Say hello to the Bullet Cluster. two
immense groups of galaxies billions of
light years away that offer some clues
in the dark matter mystery.
So the bullet cluster is actually made
up of two separate clusters of galaxies.
About 100 million years ago, gravity has
pulled them together and smashed them
into each other. So in this in this
collision we have these these two types
of matter. We we have the galaxies and
and we have the 100 million degree gas
which sits between all the galaxies. 100
million degree gas. Yeah. So, what you
might be asking, do the galaxies and gas
of the bullet cluster have to do with
dark matter?
Let's create a mini bullet cluster
collision to demonstrate. And here to
help is Neil deGrasse Tyson.
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[Applause]
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1 2 3.
So when we take these two separate
clusters of galaxies in the in the
bullet cluster and smash them together,
the galaxies continue straight on
without without really any effect. The
gas clouds however are going to behave
just like any other sort of gas. And so
they actually do collide. This is going
to slow them down. And so now when we
look at this cluster about 100 million
years later, the galaxies are moving
faster than the gas clouds. And so
they're going to be further away from
from the center of this collision.
In the center of this collision are the
intergalactic gas clouds lagging behind
the faster moving galaxies.
The the galaxies are now in a physically
separate part of the sky from what the
gas is. And so we've separated out the
two types of matter which we can see.
Now here's the important part. Matter
creates gravity. More matter, more
gravity.
90% of this matter is in the gas cloud.
And so that is where the gravity should
be.
But when all the data was put together,
that wasn't the case.
Instead, we see that most of the gravity
is actually around the galaxies. And
that tells us that something else has to
be there causing this gravity. It can't
just be the stars in galaxies
themselves.
So, is this evidence for dark matter?
Let's ask three top experts in the
field.
There is clearly some stuff there that
that is not normal matter and that's
making a lot of gravity.
I I really believe that there really
there is some dark matter there.
So I think the bullet cluster really is
the silver bullet that kills off a lot
of theories trying to explain away dark
matter.
Although the dark matter mystery isn't
completely resolved, we may have good
reason to be grateful for this
mysterious stuff.
Dark matter is basically the glue that
holds all the large structures in in the
universe together. So without dark
matter, we would not currently be here.
So three tiers for dark matter, whatever
it may be.
For more on dark matter, tune in to Nova
Science Now airing in the summer of
2008. In the meantime, you can find
other fun science stories on our website
at pbs.org/nova/science
now.