Section 3.3.1: Why do galaxies collide?
The Universe is a vast place. Although it is filled with over a hundred billion galaxies, many of these are separated by unimaginable distances. Not only that, but the Universe is expanding, as we will explore in Section 5. The expansion of the Universe carries the galaxies along with it and, in general, increases the distances between galaxies. Bearing all of this in mind, how can two galaxies collide together?
The answer is once again due to gravity, and also the fact that galaxies are not evenly distributed throughout the Universe. Instead, you find some areas where there are lots of galaxies (groups and clusters) and some areas where there are no galaxies (voids).
ACTIVITY: This can be seen in the below video which shows the positions of 400,000 nearby galaxies. As the video flies through the galaxies, see if you can spot some clusters and voids.
The overall motion of one galaxy relative to another is a result of the competition between the expansion of the Universe (moving them apart) and the mutual attraction of gravity (pulling them together). If two galaxies are close enough then gravity will win and the galaxies will eventually crash together.
Note that not all galaxy interactions result in a collision, and not all collisions result in a merger. It all depends on the relative velocity and distance of the galaxies.