Black holes come in lightweights, born from dying stars, and supermassive ones at galactic cores. The missing intermediate link had been hiding. Fast radio bursts, cosmic signals lasting milliseconds but carrying the energy of days of sunlight, helped crack the case. Think of these bursts as cries from the depths of the universe. When such a cry skims past a massive invisible object, its path bends, spacetime curvature kicks in, and we receive an altered signal. Astronomers studied the frequency prints of two such 'mumbling' bursts and discovered: objects with masses from 500 to 2,500 Suns blocked their path. Without companion stars and far from galactic clusters, they are prime candidates for intermediate-mass black holes. Here's the twist: if no galaxies are nearby, these invisible giants could have been born not from stars, but right after the Big Bang, as primordial black holes. Stephen Hawking suggested they could account for dark matter. Calculations show such holes make up at most 4% of dark matter, but if the signals mislead, the limit rises to 13%. In any case, we are hearing whispers from the first moments of existence.
A fast radio burst unleashes more energy in a millisecond than the Sun does in a week.
In sci-fi, primordial black holes are portals to other universes. But their real role may be humbler: invisible bricks of our cosmos.