Right after the Big Bang, the Universe expanded incredibly fast — this phase was called inflation. In some microscopic regions, matter randomly clumped so much that it collapsed into primordial black holes. They are tiny — the size of a proton, yet as massive as an iceberg.

Scientists were tormented by the question: wouldn’t these turbulent small-scale processes spoil the delicate picture of large-scale waves we observe in the cosmic microwave background? Alan Guth soothed: the basses and flutes in the cosmic symphony play at different frequencies and don’t interfere with each other. New calculations proved that any back-reaction is a thousand times weaker than the sensitivity threshold of our instruments. The melody of the cosmos remains pure.

Thus primordial black holes earned the right to be considered part of dark matter — the invisible substance that holds galaxies together. The irony is that, according to Stephen Hawking’s calculations, ordinary black holes evaporate, while these long-lived ones will exist forever.