The further we get from a galaxy, the lower the density of intergalactic gas, to almost imperceptible values. To understand this emptiness, it is enough to compare our atmosphere with the environment between galaxies. The air we breathe contains 5×1019 atoms per cubic centimeter, that is, 50 trillion; while in intergalactic space the equivalent figure is 0.000001 atoms. This means that each atom lives freely in a cube that is one meter on a side.
Now, this apparently insignificant figure becomes -and never better said- astronomical when the immense space that separates some galaxies from others comes into play. Cosmologists estimate that the total atomic mass of intergalactic gas exceeds that of stars and galaxies together by 50 percent. That is why scientists are more aware of the so-called empty space to understand the evolution of the universe, from its started with the big bang, the Big Bang , until today. But the study of intergalactic gas, so thin and dark that it does not produce light , is very complicated. In general, its detection is done indirectly by analyzing the effect it produces on radiation from distant sources, in the case of quasars, galaxies with a black hole in the center.
