Q&A: Galaxies, Galaxy Clusters, AGN, and Quasars
If the universe is expanding (because of the "dark energy" forcing things apart), then why do we see so many galaxies colliding with each other? Even the Andromeda Galaxy is going to collide with our very own Milky Way Galaxy.
The universe is not only observed to be expanding, but this expansion has been observed to be accelerating. The name "dark energy" has been given to the mysterious energy that is driving this acceleration. Although the origin of this dark energy is unknown, astronomers know quite a lot about the properties of dark energy, including the physical scales over which it acts. On the scales of galaxy clusters, the attractive force of gravity provided by matter (especially dark matter) will dominate over the repulsive effects of dark energy. So galaxies that aren't too far apart, like the Milky Way and Andromeda, will collide with each other over the next few billion years. But on larger scales the repulsive effects of dark energy will dominate, driving galaxies apart so that they never collide with each other. For example, the merged galaxy that forms when the Milky Way and Andromeda collide - called "Milkomeda" by some astronomers - is not expected to ever collide with M87 or any other galaxies in the Virgo Cluster, the nearest galaxy cluster to us. Without the effects of dark energy such a collision would have been inevitable.