Collision of Galaxies
The expansion of space is something that happens on the largest scales.  At small scales, such as distances between nearby galaxies, other forces, such as gravity, dominate.  Galaxy clusters are held together by the attractive force of gravity between these galaxies.  Space in these regions is still expanding, but the gravity pulls on these galaxies much more than space's expansion moves them apart.

Galaxies that are close enough together, such as the Milky Way and Andromeda, are pulled together by the force of gravity between them.