A few hundred million years after the Big Bang, the universe resembled an endless dark ocean, shrouded in a thick fog of neutral (non-ionized) hydrogen. Then something dispersed the gloom: ultraviolet radiation knocked electrons out of atoms, making the gas as transparent as pure water. This era is called

For a long time, it was thought that the main illuminators were young galaxies filled with hot stars. But new research puts the spotlight on supermassive black holes — active galactic nuclei. Updated models showed that these cosmic beacons emitted 3–4 times more ionizing light particles than previously estimated. With this, their contribution to clearing the universe reached 20%, and that's without counting the dimmest but most numerous black holes. Astonishing: the darkest objects in space turned out to be brighter than hundreds of millions of Suns.