Question: how would you explain dark matter?

Jack Miller answered on 26 Jun 2013:

Hi Sam,

Dark matter is matter that we cannot see, but know that it must be there. It’s highly likely that dark matter is made up of ‘weakly interacting massive particles’ or WIMPS that are very difficult to experimentally detect. A WIMP would only ever interact with anything else by the weak nuclear force — one of the four fundamental forces in nature that is ordinarily only found in certain types of radioactive decay. The weak force is, unsurprisingly, very weak compared to electromagnetism or the strong nuclear force, but not as weak as gravity. It’s therefore really, really hard to try and find a WIMP, as you need a detector with a huge volume to maximise the chance that it interacts weakly with something you can then detect.

We know that dark matter is out there because we can see it’s effects gravitationally — galaxies often have shapes and properties that only make sense if there’s more matter out there than what we can detect electromagnetically. We can even map the missing mass, by working out where it should be. The conclusion we come to is that there’s a LOT of matter we can’t see — which strongly implies that there are a lot of WIMPs we can’t detect.

Hope that helps!

— Jack