From the January 2010 issue

How is it that galaxies merge and collide but the universe is expanding and everything is moving away from everything else?

Justin Hammersley, Sterling Heights, Michigan
By | Published: January 25, 2010 | Last updated on May 18, 2023
Merging galaxies
The Milky Way and Andromeda galaxies will one day collide despite the fact that space-time itself is expanding. That’s because local gravity overpowers the expansion and pull these galaxies toward one another.
Astronomy: Roen Kelly

Whether or not galaxies merge depends on how strong the gravitational attraction is between the galaxies and whether the universe’s expansion is more powerful than gravity.