Dark Era of the Universe: Is it Really Dark?

[jordankonisky]

I know that the dark era in the history of the universe is considered dark because stars had not as yet formed and so there was no light/photon production. Nevertheless, there were present those photons that we observe today as the CMB radiation. So when we refer to that period as being dark, do we really mean the absence of light (photons) at wavelengths that are in the human visible spectrum?

 

[phyzguy]

Not really. The CMB photons simply streamed through those regions. So we really mean that no new photons were added during that period.

 

[Javier Chornet]

In the epoch of the atomic formation (that finished when the CMB light was emmited) all the energetic photons was absorved by the electrons of the atoms (basically hydrogen), and the only photons that was not absorved was these whose energy at these moment was lower than the first energy of hidrogen ionization (5,87433 eV). That are microwaves, and these microwaves are what we see of the CMB (now the energy is lower because of the expansion of the universe), so during the dark epoch these photons existed.
As you said, the name is because in our wavelenghts we can't see anything at these moment (just the beginning: the CMB) because there was not any other elecromagnetic radiation (as produced by stars). So the end of that epoch is when first stars are formed.