In this video we'll discuss the
mysterious nature of dark matter. So far
we've learned about ordinary matter - 
stuff like apples, cars, quad bikes, sheep,
planets and stars. Looking up at the
night sky, it looks like most of the
matter in the universe is stars, planets
a fair amount of intergalactic dust and
sheep. But actually physicists are now
pretty certain that everything we could
possibly see and touch in the universe
is only about 15% of the matter. The rest
is something known as dark matter. Back
in 1939, Horace Babcock was completing
his PhD thesis. He was looking at spiral
galaxies which contain a central core of
lots of stars and arms of stars
extending outwards. These stars should be
rotating about the centre of the galaxy
which they are and, like planets in the
solar system, the rotation speed should
decrease as you move away from the
centre of the galaxy. Here's a graph of
the predicted rotation speed as a
function of distance from the centre of
the galaxy. And here's the measured
rotation speed as a function of distance.
The rotational speed stays the same as
you move further out, so something twice
as far away will move just as quickly.
But this doesn't make sense.
According to Newton's law of universal
gravitation, we would expect it to be
decreasing in speed. If we can see all
the matter then we should be able to
predict how it will move with Newton's law of
universal gravitation. So Babcock made a
perfectly reasonable inference. Either some
of the light from the galaxies was
blocked and there was more matter than
we could see, or we were fundamentally
wrong about how gravity works at large
scales. However there was a third
possibility that Babcock either didn't
think of or didn't have the audacity to
suggest: that our models and
understandings are correct, but there's
extra invisible matter that we can never
hope to see which affects the dynamics. 
This invisible matter is commonly called
dark matter and it's the leading
contender for explaining our
observations. Now if this feels like
cheating to you, rest assured most people
feel that way when they first learn
about dark matter. And in fact there's
even an xkcd on this.
But xkcd really is right. There's a huge
number of strange observations at
galactic scales and dark matter explains
these observations really well to the
extent that there is a near consensus
among astronomers that most of the
matter in the universe is dark. So what
is dark matter? Well, as the name implies
it's dark. It doesn't experience
electromagnetic or strong nuclear
interaction, otherwise we would have seen it. That means it's entirely possible
that there is dark matter streaming  through the room you're sitting in right now. So
why don't we walk around getting
buffeted to and fro by all the dark
matter everywhere? Well, when we say it's
dark, we mean it doesn't interact via any
force except for gravity and perhaps the
weak nuclear force. So any push or pull
from dark matter on us would be
undetectable. We call particles that
don't interact with other matter except
through gravity or the weak force, weakly
interacting massive particles or wimps.
There are currently experiments underway
looking for wimps, but by their nature
they will be difficult to detect. Some of
the experiments are looking for direct
collisions with nuclei while others are
looking for the byproduct of wimp decays or annihilations. If they even do
decay or annihilate, this is an active
area of research for physics so
hopefully in a few years we'll know more
than we do now.
