End of Cosmic Dark Ages: Universe Becomes Transparent

Approximately 500-800 million years after the Big Bang, the universe emerged from the Cosmic Dark Ages—a period when neutral hydrogen gas prevented light from traveling freely through space. As the first massive stars and proto-galaxies formed, they emitted intense ultraviolet radiation that gradually ionized the surrounding hydrogen gas through a process called reionization. This transition fundamentally altered the universe, making it transparent to light and enabling the cosmic structures we observe today. The end of the Dark Ages marks a critical boundary between the opaque early universe and the transparent universe we can directly observe with telescopes.