Stellar-mass black holes, which contain roughly 5 to 20 solar masses, often give themselves away when they are part of closely separated binary systems. Infalling matter from their companion star piles up in a hot accretion disk, which can produce outbursts of X-rays.

Using NASA's Chandra X-ray Observatory, Muno's team caught seven of these outbursts over the past five years. Each was found within 75 light-years of the Milky Way's center, and surprisingly, four of them occurred within just 3 light-years of the core.

Given the known X-ray behavior of these objects, this high spatial density suggests there are tens of thousands of black holes swarming in that small region of space — some 20 times more holes than expected.