Question: What is Dark Matter and why is it a big deal?

Ian Cade answered on 17 Nov 2014:

If you look out at the night sky you can see stars, if you look through a telescope you can also see lots of galaxies… some of which look like spinning spirals, like our own galaxy.

If you work out how fast these galaxies are rotating (from doplar measurements on the light they are emitting) and estimate how much matter there is in the galaxy (count up all the stars, dust clouds etc) you end up with a result that means there isn’t enough matter holding the galaxy together via gravity (and the galaxy should just fly apart)…

Evidently the galaxy hasn’t flown apart because its up there and spinning fine… So, this must mean that there’s extra mass that wasn’t accounted for in our estimates (which are based on stuff we can see, so the ‘missing’ mass is labelled ‘dark matter’ in that we can’t see it)

So, what might this dark matter be?

It could just be old dead stars that have stopped glowing so we can’t see them.

Alternatively it could be due to some unknown particles that are very difficult to detect

So, why is it important?

It turns out that the composition of the universe is about 5% matter 27% dark matter and 68% dark energy (i.e. we only have any idea about the material that makes up 5% of the universe!)… so dark matter is important simply because there’s a lot of it about.

But also, if it is made of undiscovered particles, this is also important because present theories that describe how subatomic particles interact (and create everything we know about) work well, but we know they are incomplete… so, a new experimental observation of a new type of particle might give some hint how to improve these theories.