Black holes aren't silent abysses, but nature's tuning forks. Their rapid spin, predicted by Roger Penrose, makes spacetime itself sing, producing a stream of the lightest particles — axions. These accumulate around the hole, like sound in a concert hall, and begin to oscillate in unison. Thus a cloud forms, radiating gravitational waves of a single unchanging note — a cosmic tuning fork sounding for millions of years.

Previously, scientists tried to catch such a song from individual holes by monitoring their spin. Now they've taken into account that in our Milky Way alone there are about a hundred million such tuning forks. Their combined choir produces a hum that detectors built by Rainer Weiss (LIGO) can already pick up. The next generation of instruments, like the Einstein Telescope, will be able to distinguish notes from even lighter axions, bringing us closer to solving the dark matter puzzle.