Detectors of gravitational waves caught the merger of two unusually heavy black holes — event GW231123. Their mass is too large to come from ordinary stars. Scientists believe they grew from tiny seeds that appeared right after the Big Bang. This idea of primordial black holes was proposed by Stephen Hawking.
The growth of such a hole is like a whirlpool in a sink: the more matter it sucks in, the more massive it becomes and the faster it spins. Over billions of years, dust and gas spun it up to enormous speeds, explaining both the mass and spin of GW231123.
Primordial black holes could have formed from random clumps of energy in the first second after the Big Bang.
In science fiction, tiny primordial black holes are sometimes depicted as invisible 'bullets' that can pass through planets and cause disasters.