Black holes are, by definition, invisible. But the region around them can flare up periodically when the black hole feeds. As gas falls into a black hole, it will heat to high temperatures and radiate brightly, particularly in X-rays.

Thanks to ESA’s XMM-Newton data, astronomers found one stellar-mass black hole by chance feeding in a globular star cluster in a galaxy named NGC 4472 (or M49), about fifty million light-years away in the Virgo Cluster.