Question: If axions could be a part of dark matter, yet we can't see them, how do we know that they have a mass? If it turns out that they don't exist, what will we think dark matter's made up of?

Francesca Day answered on 13 Mar 2014:

Great question! You are right, axions might not exist, or they might exist but have little or no mass – we just don’t know at the moment! Although we can’t see axions, we might be able to work out whether or not they have mass because how heavy they are affects how they interact with photons (particles of light).

There are lots of different theories for what dark matter is, and scientists argue a lot about it! One popular theory is called supersymmetry. Supersymmetry says that every particle in the Standard Model (these are all the particles that have been discovered so far) has a much heavier “super twin”. One of these super twins could make up dark matter.

There are lots of big experiments trying to work out what dark matter is at the moment so hopefully we will get there soon 🙂 Or maybe one of you guys will figure it out!