Galaxies and their central supermassive black holes grew in tandem, the result of countless collisions and mergers between ancient, smaller galaxies. But galaxies sometimes merged without combining their black holes, ejecting some of these objects out into the depths of open space.
According to a computer simulation by Valery Rashkov and Piero Madau at UC Santa Cruz, a shocking number of these abandoned black holes might be found in the Milky Way's halo, which is a giant outlying region of gas found beyond our galaxy's stars. There's considerable variance in terms of just how many black holes are out there — Rashkov and Madau place the number of black holes between as low as 70 and and as high as 2,000.
These objects are what the researchers refer to as "seed" black holes. These intermediate-sized black holes were once found at the center of early collections of stars and gas — these structures weren't big enough to be considered galaxies in their own right, but they combined as the building blocks for galaxies like the Milky Way. While a good number of these original, relatively small black holes would have merged together to form the current crop of supermassive black holes, but the chaos of these intergalactic mergers could have left some of the black holes stranded in the far regions of space.
While most of these rogue black holes would be pretty much impossible to detect, some of them might have brought entire star clusters and clumps of dark matter along with them. If that's the case, we should be able to spot the light of those clusters in the Milky Way's halo. For more, check out New Scientist and the original paper at arXiv.
In lieu of a story-specific image, enjoy the above artist's conception of the IC 10 X-1 system, with the black hole in the upper left. Credit: Aurore Simonnet/Sonoma State University/NASA.