All of the matter discussed so far in this chapter only makes up 5% of the energy density of the Universe. (We can talk about mass density and energy density interchangeably, as mass is a form of energy. The amount of energy $$E$$ in mass $$m$$ can be found through the famous conversion $$E=m c^{2}$$.) Several different lines of evidence have shown us that most of the mass in galaxy clusters is not the luminous mass we can observe. Indeed, this mass can’t be baryonic at all. Dark Matter is the name given to this mass; it’s not dark the way dust is, which absorbs light is. It neither absorbs nor emits light; it only interacts with light gravitationally (and has been observed partly through the gravitational lensing effect). Dark Matter interacts only via gravity and, perhaps, the Weak Force. In this way, it is similar to neutrinos. We have not identified a particle that can make up Dark Matter. We’re very sure that it’s there, and we’re very sure that it makes up most of galaxies and galaxy clusters, but we don’t know exactly what it is.