A new theory suggests dark matter is made up of particles that strongly interact with each other via a so-called ‘dark force’. If A true, this may finally explain the extreme densities we see in dark matter halos surrounding galaxies. The existence of particles called selfinteracting dark matter (SIDM) acts as an alternative to cold dark matter theories which suggest the elusive stuff is made up of massive, slow-moving and thus cold weakly interacting particles that don’t collide. The problem with those cold dark matter models is that they
