General relativity, 6

Modern cosmography The “normal” matter in the universe—i.e., stuff made of protons, neutrons, and electrons— consists, approximately, of lumps floating in a dilute fog. The lumps are galaxies, clusters of 107 to 1011 stars bound together by gravity. In the currently observable universe, it is estimated that there are roughly 1011 galaxies. The dilute fog is primarily neutral atomic hydrogen gas with some helium-4 mixed in (making up a total of 98% or more of the fog); there are also very small fractions of 2H (deuterium), 3He , and 7Li . The ratio of hydrogen to helium is about 3:1 in terms of mass and about 10:1 in number of atoms. The total mass in the gas clouds is much larger than in galaxies. On average the density of ordinary, visible mass in the observable universe is equivalent to about one proton per cubic meter. Thus, there is almost no ordinary matter (on average) in the universe.