Dark Ages (Chapter I)

The Dark Ages starts around 400,000 years.

Post-Atoms Period
After atoms were formed, it was dark and lack of light because of no stars existing but heat cools down further with lack of heat, it's barely dark and nothing but empty for millions and millions of years.

End of the Dark Ages
The gases of helium and hydrogen is dominant with lack of lighting where it lasts for the next hundred thousand years until stars formed to create a lighter and still dark universe. At ~100-125 million years later, the dark ages ended and stars form, but appear later as there are only stars so called the Quasi Stars, for more details please go to the next character.

Metal-free stars start around ten million years later.