so let's talk about dark matter dark
matter matters pun intended remember we
had our model of the Milky Way as sort of a
fried egg so here we had the spiral arms
the disc here we have the central bulge
and here we have the globular clusters
and the old halo stars now where do you
think most of the mass of the galaxy is
well doesn't it make sense that it
should be where we see the most stuff so
most of the mass of the galaxy
the galaxy should be heaviest here in
the center because that's where we see a
lot more stuff makes sense right so
remind you of anything where we have
most of the mass in the center and then
most of the other stuff is sort of flat
and spread out that's right the solar
system most of the mass of the solar
system is in the middle in the Sun and
the planets orbit around the Sun and
then you can think of the halo here as
the Oort cloud where comets are so if you
remember the planets mercury being
closest to the Sun orbits a lot faster
than Neptune which is farther out here
I'm not just talking about how long it
takes to go around because obviously
Neptune has a farther path to travel but
if you actually measure the speed the
orbital velocity of mercury compared to
Neptune mercury is orbiting around the
Sun a lot faster you can think of it as
being pulled harder by gravity so if we
were to plot this for our solar system
distance away from the Sun versus how
fast the planets were orbiting the Sun
we would get mercury very close to the
Sun orbiting very fast the earth farther
out orbiting not as fast Jupiter further
out orbiting slower Neptune further out
orbiting very slow we connect those
points see how it drops off like that
that's called
a Keplerian rotation curve Keplerian
because it follows Kepler's laws of
motion so if we were to do the same
thing with the stars orbiting around the
center of our galaxy wouldn't you expect
to get the same exact thing wouldn't you
expect that the stars here near the
central bulge would be whizzing around
really fast like mercury whizzes around
the Sun and the stars way out here near
the edge of the spiral arm they should
be going very slowly well it's not what
we found we found that the rotation
curve of the Milky Way galaxy looks sort
of like this now even with my bad
drawing I hope you can see that this is
not the same as this it's not even close
all these stars out here in the disk
they're all orbiting at about the exact
same speed one of two things has to be
true either the laws of physics have to
be changed or we were wrong in assuming
that most of the mass of the galaxy is
in the center but that's where we see
the most stuff right it made perfect
sense to assume that most of the stuff
in the galaxy was in the center of the
galaxy because that's where we see the
most stuff but what this means is that
most of the galaxy is that most of the
mass of the galaxy is not only not here
but most of the mass of the galaxy is
actually invisible that's why we call it
dark matter because you cannot see it
with any wavelength of light you can't
see it in visible light in x-rays in
radio in microwaves in anything it does
not give off light it is some form of
particle again we don't know exactly
what it is there are various
models and in science when you have
competing models we gather all the
information and then once we have enough
information will be able to tell you
exactly what it is but dark matter is
some sort of subatomic particle that
does not give off any form of light
whatsoever it's not made of planets it is
not made of dead stars it is not made of
black holes right other than that can't
tell you too much more about it we don't
know what we know that there are several
times more dark matter in our galaxy
than there is visible matter how do we
know that that's how we know that and we
also know where the dark matter is we
know the distribution of the dark matter
again how do we know that that the dark
matter is everywhere the dark matter
actually forms an additional halo
surrounding and filling the Milky Way so
the halo of dark matter don't confuse
that with the halo where these old stars
are both of those halos occupy about the
same space although the halo of Dark Matter
extends out further so the globular
clusters and the halo stars are not the
dark matter but Dark Matter fills the
space in between them
