Why are galactic collisions common even though stars are far apart?

A galaxy is enormous, and space between stars is mostly empty, so direct star-to-star collisions are uncommon. But galaxies themselves are bound by gravity and move in groups and clusters, so their orbits often bring them together. When they meet, tidal forces can stretch them, trigger bursts of star formation, and eventually merge them into a larger galaxy. The Milky Way is expected to merge with Andromeda in about 4 to 5 billion years. The result will likely be a large elliptical or lenticular galaxy often nicknamed Milkomeda.