The key idea in explaining the way in which structures evolve in the universe is gravitational instability.
[1] If material is to be brought together to form structures, then a long-range force is required, and gravity is the only known possibility.
(Although electromagnetism is a long-range force, charge neutrality demands that its influence is unimportant on large scales.)
This extra material makes them even more dense than before, increasing their gravitational attraction and further enhancing their pull on their neighbors.
It is an appealingly simple picture, rather spoiled in real life by the fact that while gravity may have the lead role, numerous other processes also have a part to play and things become quite complicated.