(interval when the universe's hydrogen atoms were neutral)
In cosmology, the dark age is the time between recombination
and the formation of stars and galaxies (producing
light),
the latter point-in-time sometimes called the cosmic dawn.
During this dark age, atoms were neutral and space was transparent but
there were no stars and all was relatively dark.
Recombination (which initiated the dark age) was when ions and
electrons formed neutral hydrogen atoms, some 380,000
years after the Big Bang, at a redshift of about 1090.
The cosmic dawn (which ended it) was essentially the beginning of
the epoch of reionization (EOR), roughly 150 million years after the Big Bang:
ionizing radiation from stars again ionized
hydrogen atoms in large numbers, this ionization occurring from
redshift 20 to redshift 6.