It is common for galaxies to collide with each other. A galaxy has about 100 billion stars in it. You would probably think that the stars in the different galaxies would collide but the chance of that happening is almost a 0 because the stars are too small to hit each other and has average distance between them. This isn't how we observe galaxies interacting. This is because the universe is filled with gas and stars. When the galaxies collide there is friction happening and that causes a shock wave that triggers some of the stars. When two galaxies collide with each other they start forming into one and become bigger. Astronomers think this is how the Milky Way got bigger and bigger.