The black holes are crash victims, ejected from their original host galaxies when worlds collided, a process that Ryan O'Leary and Abraham Loeb, with the Harvard-Smithsonian Center for Astrophysics, suspect was instrumental in building our own galaxy and probably many others.

"Our work was theoretical, but we have an idea of what these clusters would look like, Loeb told Discovery News. 

"Technologically, it's not difficult to detect these clusters," said Loeb. "The issue is really finding the needle in the haystack."

The research, which is to be published in Monthly Notices of the Royal Astronomical Society, builds on computer simulations showing how much energy would be needed to liberate a black hole from its host galaxy.

Manuela Campanelli at the Rochester Institute of Technology in New York said the specifics depend on the size of the colliding black holes, the angles at which they meet and their spin rates. The most dramatic crash would be a collision between two similarly sized black holes, which would send them careening across space at a velocity of 2,500 miles a second.

"That was a shock to the astronomical community, Campanelli told Discovery News.

The calculations show that black holes can easily be ejected from their host galaxies, she added.

O'Leary and Loeb believe this was common practice in the formative years of the Milky Way, as small dwarf galaxies crashed into each other. The ejected black holes would not have enough velocity to escape the gravity of the newly combined mass and should still be wandering the outer regions of the galaxy today.

"An observational discovery of this relic population ... would constrain the formation history of the Milky Way and the dynamics of black hole mergers in the early universe, the astronomers wrote.