Astronomers believed a "supermassive" black hole sits at the centre of the Milky Way, called Sagittarius A*. But if you took the equivalent space around Earth there would be zero black holes, not thousands, Hailey said.
For some time, the theory has been that the supermassive black holes at the centre of most galaxies, including our own Milky Way, are accompanied by smaller black holes. And because scientists know what fraction of black holes will bind with low mass stars, after they discovered the 12 they were able to calculate the population of isolated black holes around Sgr A*. "Theory is a lot more interesting when you have observations to compare against".
So to determine the number around the Galactic Center, the researchers had to delve into their knowledge of black hole physics.
Researchers led by Columbia University published a study in Nature that shows that there are around a dozen black holes surrounding Sagittarius A*, which is the supermassive black hole at the center of the Milky Way. "These objects also provide a unique laboratory for learning about how big black holes interact with little ones, because we can't readily study these processes in other, more distant galaxies".
"Isolated, unmated black holes are just black - they don't do anything", said Prof Hailey. For perspective, apart from these newfound dozen scientists have only identified about 60 black holes in the entire Milky Way, and all but a few are far from the galactic center.
Instead, they sniffed out the steady X-ray emissions expected from the disc of gas and dust that swirls around a black hole.
Then they analysed the properties and spatial distribution of these binaries, and extrapolated on these findings to conclude that there must be somewhere between 300 and 500 low-mass binaries in the Milky Way's central parsec, and around 10,000 lone black holes.
What Hailey finds particularly exciting is that the new findings could also aid in more discoveries and the study of gravitational waves, first seen in 2015.
Scientists said it could enhance our understanding of gravitational waves, the ripples in space-time created by powerful cosmic events such as when massive objects collide.
"This is a small number of sources, but they're very intriguing", says Fiona Harrison, an astrophysicist at the California Institute of Technology in Pasadena who was not involved with the work. "A discovery like this will always have consequences that we can not presently predict", he says. This will naturally produce copious neutron stars (very dense stars) and black holes, which can form close "binary systems" in which a normal star and a neutron star or a black hole orbit each other.
There are good reasons the Milky Way's black holes tend to be in the center of the galaxy, Hailey said. Mysteriously, most of these black holes are inconveniently sized, appearing too large to have readily formed directly from dying massive stars. A black hole that isn't eating appears as a single dark dot against an equally dark background, and as such, is nearly totally invisible.
You can't see an inactive black hole, one that isn't consuming anything.
The researchers found 12 black holes with similar masses as the sun surrounding the supermassive black hole Sagittarius A* that lives in the galaxy's center. But mostly the centre of the galaxy is the flawless "hot house" for black hole formation, with lots of dust and gas.