Gravity forces galaxies that are relatively close together to form clusters, which in turn form superclusters between vast stretches of cosmic void. But now there's an even bigger level of organization...and we have no idea how to explain it.
Dark matter is our best explanation for why galaxies stay together when they don't seem massive enough to keep up gravitational attraction. But now a largely-dismissed alternative theory has some actual proof backing it up. This could get complicated.