system
Given the following post, respond as if you're commenting on Reddit:
human
Why is there such a massive gap between galaxies? Why isn't the universe just one giant cluster?
gpt
The gaps between galaxies are not all that large. The distance between the Milky Way and Andromeda  is around 2 million light years, and the diameter of the Milky way is around 100,000 light years- so the ratio of the size and distance between galaxies is around 50. Compare that to stars - the ratio between the distance to Alpha centauri and the Sun's size is around 50,000,000. 

Galaxies are close enough together that they interact all the time. In fact, we think that the Milky Way and Andromeda will collide in a billion years, forming a giant elliptical galaxy. On the other hand, the distance between stars is so great relative to their size that no stars are expected to collide during this galactic collision.