Why do galaxies collide without most stars crashing into each other?

Even though galaxies contain billions of stars, the distance between stars is enormous, making direct collisions rare. When two galaxies interact, gravity distorts their shapes, triggers gas clouds to collapse, and can ignite bursts of star formation. The Milky Way and Andromeda are expected to merge in about 4 to 5 billion years. Their stars will mostly pass by one another, but the overall galactic structure will be dramatically altered. The result after a long merger is likely to be a larger elliptical or lenticular galaxy.