How can galaxies merge if the everything in the universe is moving apart?

The Andromeda Galaxy has long been predicted to collide with our Milky Way in about 5 billion years; this illustration imagines the view as our neighbor looms large in Earth’s skies. However, a paper published in the June 2025 issue of Nature Astronomy questioned the likelihood of that merger, depending on interactions with other Local Group galaxies. Credit: NASA; ESA; Z. Levay and R. van der Marel, STScI; T. Hallas; and A. Mellinger

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

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. Gravity affects everything in the universe. The Milky Way Galaxy is gravitationally pulling on NGC 3370, nearly 100 million light-years away. The catch, however, is that the gravitational attraction between the Milky Way and NGC 3370 is so tiny that the universe’s expansion can overcome it.

Over small scales, the force of gravity can beat out cosmic expansion and hold structures together, such as galaxy groups and galaxy clusters. Our Local Group of galaxies is an example of a grouping held together by mutual gravitation. And actually, the gravitational attraction between the Milky Way and the Andromeda Galaxy is strong enough that we’re moving toward each other and may collide in some 5 billion years. 

Liz Kruesi
Science Journalist and Editor, Boulder, Colorado

This question and answer originally appeared in the March 2010 issue.