After recombination, the universe was filled with neutral hydrogen and helium. Intergalactic space was transparent to visible light, but high-energy ultraviolet photons were still absorbed by the neutral atoms — the universe was 'foggy' in the UV. This haze cleared gradually as the ultraviolet output of the first stars and galaxies built up. Around 200 million years after the Big Bang small ionized bubbles formed around individual stars; over hundreds of millions of years these bubbles merged until they covered nearly the entire volume of the universe. The process was largely complete by 1 billion years after the Big Bang (~12.8 Gya).

Reionization is not strictly an 'event' but a process — yet it is one of cosmology's key turning points. After it, the universe became almost fully transparent to both visible and ultraviolet light. This is the basic precondition for our ability today to observe distant galaxies.

Who drove reionization? This remains an open research question. Three candidates dominate: intense UV light from the first stars (especially Population III and the massive stars of early galaxies); high-energy radiation from the first quasars (luminous disks around massive black holes); and other, less well-understood exotic sources. The James Webb Space Telescope has now observed many early galaxies from the era of reionization, and they appear to produce abundant UV photons despite their small size — pushing them forward as the leading candidate.

After reionization, most of the matter in the universe returned to an ionized plasma state — for the first time since the formation of atoms 380,000 years after the Big Bang. This sounds paradoxical: 'recombination made the universe transparent, and reionization did too.' The difference is density: the cosmos was small and compressed at recombination but vastly expanded and rarefied by the time reionization completed, so this second transparency was permanent.