Anne Verhamme

A galaxy is like a house with drawn curtains: bright ultraviolet light is born inside, but from outside you only see what manages to sneak through dust and gas. Astronomers need to know how much of this light escapes into intergalactic space, because it ionizes hydrogen in the early Universe and makes it transparent.

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.