When two galaxies collide, their central black holes merge, emitting gravitational waves. Astronomers theorize that a recoil effect sometimes kicks the merged black hole out of the galaxy while dragging nearby stars along for the ride. Researchers from SRON and Radboud University have now made a prediction of what these clusters will look like to identify them and prove their existence. Their findings are published in MNRAS.
Source:
https://phys.org/news/2020-06-astronomers-composition-elusive-black-holes.html