Kanak Saha

Light from the hottest young stars usually cannot leave its galaxy: hydrogen absorbs it, like a thick curtain blocking a sunbeam. But sometimes the curtain parts, and ultraviolet photons burst into intergalactic space.

After the Big Bang, the Universe was filled with cold gas — mostly neutral hydrogen and helium. It enveloped the cosmos like a morning fog, impenetrable to light. The Dark Ages began. When the first galaxies ignited, their radiation, like a searchlight beam, started knocking electrons out of atoms.