They’re a massive X-ray source

The first observational evidence for a black hole emerged in 1971, and this too came from a binary star system within our own galaxy. Called Cygnus X-1, the system produces some of the universe’s brightest X-rays. These don’t emanate from the black hole itself, or from its visible companion star, which is enormous at 33 times the mass of our own Sun. Instead, matter is constantly being stripped from the giant star and dragged into an accretion disc around the black hole, and it’s from this accretion disc that the X-rays are emitted. As was the case with HR 6819, astronomers used observed star motion to estimate the mass of the unseen object in Cygnus X-1. The latest calculations put the dark object at 21 solar masses, concentrated into such a small space that it couldn’t be anything other than a black hole.