Dark matter: our method for catching ghostly haloes could help unveil what it’s made of

The search for dark matter – an unknown and invisible substance thought to make up the vast majority of matter in the universe – is at a crossroads. Although it was proposed nearly 70 years ago and has been searched for intensely - with large particle colliders, detectors deep underground and even instruments in space – it is still nowhere to be found.

But astronomers have promised to leave “no stone unturned” and have started to cast their net wider out into the galaxy. The idea is to extract information from astrophysical objects that may have witnessed chunks of it as they were passing by. We have just proposed a new method of doing so by tracing galactic gas – and it may help tell us what it’s actually made of.

Physicists believe that dark matter has a propensity to structure itself into a hierarchy of haloes and subhaloes, via gravity. The masses of these clumps fall on a spectrum, with lower mass ones expected to be more numerous. Is there a limit to how light they could be? It depends on the nature of the dark matter particles.