Black holes black holes are places in space with gravity so strong that even light cannot escape. They are born when a massive star explodes as a supernova. But after birth, some holes remain solitary, while others live in dense clusters and can collide, merging into a single more massive one. The big question: how to tell which hole grew on its own and which is the result of a cosmic 'encounter'? The answer came from studying their spin and mass.

Scientists, among them followers of the ideas of Vera Rubin and Rainer Weiss, used recordings of 259 merger events. They split the black holes into two groups: 'quiet' (slow spin) and 'fast'. And here they found something astonishing. If you take the masses of two quiet holes, add them up, and subtract the 5% energy carried away by gravitational waves, the resulting mass, surprise, lands exactly on the peak of the fast holes' distribution. Moreover, all the bumps and dips of these distributions match perfectly. It's like having a broken plate and finding a shard that fits another piece perfectly — no doubt, they're parts of a whole.

The discovery means that massive fast black holes are the second or even third generation, grown in dense galactic centers. And to explain their properties, there's no longer a need to invoke mysterious dark matter. Now astronomers can confidently build black hole family trees, just like a genealogy chart, only rendered in black.