In one popular concept, mysterious dark matter, which seems make up atleast 90 percent of the mass of the universe, determines the formation of galaxies.
At the end of the Big Bang, energy and matter are distributed almost uniformly. However, within this sea of uniformity, slight differences in temperature and density exist.
As the universe cools, gravity draws surrounding material, both dark and ordinary matter, into the areas of higher density.
As the clouds grow, the ordinary matter, mostly hydrogen and helium, sinks to the center of the cloud, where it heats up enough to ignite thermonuclear reactions.
The star clouds attract each other and clump into galaxies surrounded by halos of the original primordial dark matter, which remains diffuse.