Neutral hydrogen does have electrons — that’s what makes it neutral.
Neutral hydrogen strongly absorbs and scatters ultraviolet light and many other photon energies.
This trapped or absorbed photons emitted by the first stars and galaxies.
Reionization begins (~400 million years onward):
- UV radiation from the first stars and quasars ionized the neutral hydrogen.
- Once ionized, the gas no longer absorbed light so strongly.
- The universe became transparent to starlight, allowing photons to travel freely across cosmic distances.
