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 emissionsoccasionally 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 wavelengthsafter 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 speculation whether rocky planets or indeed life could have arisen briefly, since statistically a tiny part of the universe could have had different conditions from the rest, and gained warmth from the universe as a whole.

At some point around 400 to 700 million years, the earliest generations of starsand galaxies form, and early large structures gradually emerge, drawn to the foam-like dark matter filamentswhich have already begun to draw together throughout the universe. The earliest generations of stars have not yet been observed astronomically. They may have been huge and non-metallicwith very short lifetimes compared to most stars we see today, so they commonly finish burning their hydrogenfuel and explode as supernovae after mere millions of years. Other theories suggest that they may have included small stars, some perhaps still burning today. In either case, these early generations of supernovae created most of the everyday elements we see around us today, and seeded the universe with them.

Galaxy clusters and superclustersemerge over time. At some point, high energy photons from the earliest stars, dwarf galaxies and perhaps quasars led to a period of reionization. The universe gradually transitioned into the universe we see around us today, and the Dark Ages only fully came to an end at about 1 billion years.