If you want to add up the weight of all planets, stars, galaxies and other celestial bodies in the universe, it is anything but easy. If you do it anyway, you will notice that part of the matter seems to be missing: the baryonic part, i.e. made up of atoms, is incomplete. Baryons – protons or neutrons, for example – were apparently more than twice as numerous in the early stages of the universe as in the cosmos we can see today, this is known by comparing them with measurements of the cosmic microwave background. Experts call the phenomenon the “missing baryon problem.” But where are the missing baryons hidden?
Eight-billion-year-old radio flash makes missing matter visible – knowledge
