Dark matter halos are often found around galaxies, extending far past them. The dark matter fails to stay within the galaxy because it's not slowed down by regular interactions.
Also, dark matter is able to pull regular matter to be closer to it. If a galaxy's dark matter is moving too far away from a galaxy, it might pull the galaxy with it. (Or it might separate entirely, as happened with the Bullet Cluster.)
Also, dark matter is able to pull regular matter to be closer to it. If a galaxy's dark matter is moving too far away from a galaxy, it might pull the galaxy with it. (Or it might separate entirely, as happened with the Bullet Cluster.)