Observations suggesting the presence of a population of black holes at the centre of the Milky Way are reported in this week’s Nature.

It has been predicted that the supermassive black holes near the centres of large galaxies are surrounded by clusters of stellar-mass black holes. However, previous searches of the Galactic Centre, where the nearest supermassive black hole Sagittarius A* (Sgr A*) resides, have been unsuccessful.

Using archival data from the Chandra X-ray Observatory telescope, Charles Hailey and colleagues report the discovery of a dozen inactive low-mass X-ray binary systems - binary star systems in which one of the components is a black hole - within a parsec (approximately 3.3 light years) of Sgr A*. The authors suggest that the properties and spatial distribution of these X-ray binary systems point to a total population of hundreds of black holes associated with binary systems within a parsec of the centre, and many more isolated black holes.