The early Universe was like a construction site where, instead of slowly building floors, they used prefabricated blocks. These blocks were primordial black holes, born in the first fraction of a second after the Big Bang. They likely also serve as dark matter — the invisible substance whose gravity holds galaxies together. Such holes can be astonishingly tiny: with the mass of an asteroid, they're smaller than an atomic nucleus, so they pass right through us unnoticed, but in clusters, they act like balls rolling into a funnel, merging at the center and quickly building a giant black hole. This solves the mystery of why the Webb telescope sees massive black holes in galaxies only 400 million years old.