system
Given the following post, respond as if you're commenting on Reddit:
human
Was the cosmic microwave background once visible light?

From what I understand of the CMB it was initially very high energy and high frequency and as the universe cooled and expanded the light was shifted down to microwave radiation. Does that mean that at some point in the history of the universe the night sky was lit up in a specific color of light that we would be able to see? 

And going further was that point in time a time when there were solid planets and the universe pretty much as we know it now?
gpt
Shortly after the Recombination era - the era from which cosmic background radiation originates - the cmb was bright orange. 

To understand what exactly happened, let's backtrack a bit.
Until 380 000 years after the big bang, the universe was too hot to form neutral atoms. Electrons and protons whizzed through space which such high speeds, that electromagnetic attraction wasn't strong enough to bind them together. Thus, space was filled with a hot plasma, that gave off thermal radiation. During this time, the universe was opaque to photons. Photons would bump into electrons frequently. This scattering of photons results in a mostly in-transparent universe with a visibility of a few thousand lightyears at most. (Which sounds a lot but is very small at cosmic scales)

Due to the expansion of the universe, this plasma finally cooled down to around 3000K. Which is cool enough to form neutral atoms around 400 000 years after the big bang. Since neutral atoms don't interact with photons that willingly, the universe became transparent. Thus, the thermal radiation of the plasma in the recombination era was emitted into all directions and from all points in space. And since the plasma was around 3000K at that time, and the black body radiation at that temperature is orangy, the hole universe was filled with orange light for a few million years.

The universe during the first few million years after the big bang saw the first stars being formed, but during this time, the only radiation emitted was the hydrogen line.

Over time, as the universe continued to expand, the cmb got redshifted. Now, ~13 billion years later, the cmb is redshifted down to around 2.7K, which is in the microwave part of the spectrum.