Dark Ages and large-
Dark Ages and large-scale structure emergence , from 377,000 years until about 1 billion years. After recombination and decoupling , the universe was transparent but the clouds of hydrogen only collapsed very slowly to form stars and galaxies , so there were no new sources of light. The only photons (electromagnetic radiation, or "light") in the universe were those released during decoupling (visible today as the cosmic microwave background ) and 21 cm radio emissions occasionally emitted by hydrogen atoms. The decoupled photons would have filled the universe with a brilliant pale orange glow at first, gradually redshifting to non-visible wavelengths after about 3 million years, leaving it without visible light. This period is known as the Dark Ages . Between about 10 and 17 million years the universe's average temperature was suitable for liquid water (273 – 373K) and there has been specula...