Galaxies Are Mostly Empty Space

The Colliding Spiral Galaxies of Arp 274

When two galaxies collide, the stars that compose them usually do not. This is because galaxies are mostly empty space and, however bright, stars only take up only a small fraction of that space. But during the collision, one galaxy can rip the other apart gravitationally, and dust and gas common to both galaxies does collide. If the two galaxies merge, black holes that likely resided in each galaxy center may eventually merge. Because the distances are so large, the whole thing takes place in slow motion -- over hundreds of millions of years. 

https://www.nasa.gov/mission_pages/hubble/multimedia/arp274.html