In a perfectly uniform universe, stars and galaxies would never come to be. But even as a newborn less than 1 million years old, the universe was liberally seeded with small fluctuations in its density. Under the inexorable pull of gravity, those fluctuations formed ever-larger collections of celestial objects as material accreted onto growing structures. Galaxies mark strong concentrations of matter, locations where the seed fluctuations have grown to form so-called halos of dark matter whose mass is 1010–1014 times that of the Sun; the baryons (protons and neutrons) that form stars congregate within those halos. On large scales, the pattern of galaxies matches that of the seed perturbations. On small scales, local dynamics have a significant effect on the distribution of galaxies.
The pattern of seed fluctuations is not random, but instead depends on physical processes that occurred in the early universe. One of those processes...
