If space is expanding, why do galaxies collide?

Hi, my name is Lyn Neale, and I’ve always wondered if the Big Bang theory is correct, how can there be collisions of galaxies in space?

Lyn, the Big Bang theory proposes that billions of years ago, the universe exploded from a tiny point. The first rapid expansion, which scientists call inflation, occurred in a fraction of a second. After inflation, the universe expanded more slowly. At this point there were no stars and no galaxies.

But when the galaxies did form, they formed in clumps — or clusters — whole neighborhoods of galaxies. These galaxy clusters are still racing away from each other.

But inside each cluster, the gravity of each galaxy tugs and attracts other nearby galaxies. Imagine a schoolbus full of kids. If you look at the big picture, the bus is driving away from the city center. But inside the bus, the bully in the back has no problem pushing his seatmates to the floor or grabbing someone’s ponytail.

Likewise, galaxy clusters can speed away from each other while individual galaxies still collide and merge.

What you have in your mind?