At the center of our galaxy is a super massive black hole named Sagittarius A*. And it may not be alone.

According to their research, which was just published in Nature, NASA’s Chandra X-ray Observatory has found evidence to support the theory which says as many as 20,000 stellar-mass black holes could have drifted toward the galactic center.

In these “X-ray binary” systems, the black hole — or neutron star — pulls gas from its companion star, and that gas falls onto the accretion disk around the black hole. Chuck Hailey of Columbia University in New York led a team of researchers who used Chandra data to search for X-ray binaries containing black holes near Sgr A*, analyzing X-ray spectra within about 12 light-years of the super massive black hole.

After shortlisting and eliminating candidates — based on data from known X-ray binaries and neutron stars — Hailey and his collaborators concluded that a majority of these dozen X-ray binaries are likely to contain black holes. The amount of variability they have shown over timescales of years is different from that expected for X-ray binaries containing neutron stars.

This population of black holes with companion stars near Sgr A* could provide insight into the formation of X-ray binaries from close encounters between stars and black holes. An even larger population of stellar-mass black holes without companion stars should be present near Sgr A*. According to theoretical follow-up work by Aleksey Generozov of Columbia and his colleagues, more than about 10 000 black holes and as many as 40 000 black holes should exist in the centre of the galaxy.