Question: what is dark matter?

Ben Cropper answered on 11 Mar 2020:

I answered a similar question already, so I’ll copy my answer from there:

Not my area, but I do know kind of where we’re up to on this one – I went to a lot of these talks as a member of the university’s astronomy society!

When really big things in space interact, they do it with gravity, because they’re all so heavy. We can see galaxies spinning, moving round each other and smashing into each other. The trouble is that these galaxies look heavier than they should, and in the wrong places. We add up all of the light we see, and take into account all of the gravity we’d expect to see from that light, and we see that the outside of galaxies are much heavier than we’d expect.

Therefore, there must be something that we can’t see that is doing this! That’s what we call dark matter. Problem is, we can’t see it, so we don’t know what it is. One possible explanation is just that there’s loads of black holes everywhere (there aren’t enough stars to make these black holes, so if these existed they’d have to have been formed at the start of the universe). Another is that our theory of gravity is wrong and there actually isn’t any dark matter (we keep testing this and it keeps working, so this looks unlikely). The one that people seem to like the most is that there’s another type of particle that only interacts with anything else through gravity (and a bit through a different force, the weak nuclear force). Still, we’ve never seen one (people are looking!) so we don’t know!