"Galactic mergers" refers to the coming together of two or more galaxies in space. It happens when galaxies get close enough to each other and their gravitational pull causes them to merge into a single larger galaxy. Full definition
The multiple rounds of galactic mergers that have since taken place have given us hefty galaxies like our Milky Way and nearby Andromeda. (discovermagazine.com)
«Best evidence yet for galactic merger in distant protocluster.» (sciencedaily.com)
Perhaps, after all, a relatively recent galactic merger is responsible for Andromeda's structure — and the structure of countless other galaxies — as well. (scientificamerican.com)