Why do galaxies collide if the universe is expanding?

Question

Levi Acosta · Accepted Answer

Galaxies tend for form gravitationally bound groups.

The universe is expanding, but this simply means that larger structures are becoming more distant from one another. Galaxies can form larger structures like clusters and super-clusters, but they mostly form into gravitationally bound groups. Galaxies can collide within groups. The Milky Way galaxy and the Andromeda galaxy are on a collision course that will result in a collision in approximately 4 billion years. The Milky Way galaxy is a member of the local group, which also includes the Andromeda galaxy.

Nova Adams · Answer

undefined The universe may be expanding, but at smaller scales, gravity overcomes the expansion, leading to galaxy collisions. Over time, gravitational attraction between nearby galaxies can pull them in closer together until they collide.

Claire Anctil · Answer

undefined Galaxies collide despite the universe expanding because gravity can overcome the expansion on a local scale. In regions where galaxies are close enough together, their gravitational attraction can pull them towards each other, leading to collisions and mergers. This process is particularly significant in areas where galaxy clusters or groups are present, where gravitational interactions dominate over the overall expansion of the universe.