This simulated image shows a massive black hole at the center of a galaxy. Some massive black holes may be the result of mergers between the black holes hosted by two or more galaxies. [NASA, ESA, and D. Coe, J. Anderson, and R. van der Marel (STScI)]

An Emerging Question 

Two galaxies, adrift in the universe, pass near one another. If they become gravitationally entangled, the billion-year process of merging begins as they gradually coalesce into a single galaxy. ….

more: aasnova.org/2021/10/11/merging-black-holes-vs-gas-and-stars/