If galaxies are moving farther apart, why are there still galactic collisions?
JOSHUA COLBERT, SILVER SPRING, MARYLAND
The expanding universe is just the playing field where galaxies interact with each other. The most important way they interact is by gravity, whose range is, theoretically, unlimited. Every galaxy attracts every other galaxy, and their mutual pull slows the rate of moving apart from each other. The closer two galaxies are to each other, the stronger their gravitational attraction is (and the slower the pace at which expansion would tear them apart).