[Martin]

according to the standard picture space started out full of hot plasma and about 380,000 years from the start of expansion it cooled enough to become largely transparent neutral gas.

the CMB dates from that moment when the material filling space became transparent. the photons from that moment are still traveling

[side comment: if space were finite (analogous to the surface of a balloon) and small enough then some of them might have been around once already and be coming around again. but I dont think that's very likely. even if it is finite most astronomers think space is too large for that to have happened]

anyway all space is filled with the CMB light which used to be the heat glow of a 3000 kelvin plasma, so it would look approximately like SUNLIGHT
or a bit redder because the sun is more like 5000 kelvin
so if something hadnt happened, THE WHOLE SKY WOULD LOOK LIKE THE FACE OF THE SUN

like you copied the face of the sun hundreds of time and pasted it all over the sky until ALL the sky was as bright as the sun when you look at it (a bad idea without dark glass protection)

In fact that is what the sky looks like, the CMB light is coming from all over the sky from every direction, except for one thing--------it is the original photons from the 3000 kelvin hot plasma, but their wavelengths have all been stretched out.

these photons have been studied a lot by satellite observatory and they are coming from every direction and going towards every direction, equally at every point in space as far as we can tell (having put up detectors at various times and locations)

the only explanation that has been offered, that I know of, for why we are not at this moment burning up with 3000 kelvin radiant heat is that
the CMB photons have all had their wavelengths stretched out by a factor of 1100

So that is the main basic reason why the night sky is dark.

People give a lot of reasons for the darkness of the night. So many that it can get to sound complicated. Wondering about this goes back several centuries and is sometimes called Olber paradox although Olber was not even the first to discuss it. All the historical discussion that has built up makes it seem more complex an issue than it really is.
OK so in some cases with an infinitely old static universe (which we dont have) then even STARLIGHT would be burning us up because it would have had time to reach us from every direction in the sky. But that is largely inessential centuries old speculation.
OK so it is possible under other unreal assumptions that other stuff could be burning us up. OK.
But the main point is that if the universe had not expanded 1000 fold since they year 380,000 then the cosmic microwave background would be the light from a 3000 kelvin plasma and we would be toast.

the night looks black because space expands

[swansont]

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anyway all space is filled with the CMB light which used to be the heat glow of a 3000 kelvin plasma, so it would look approximately like SUNLIGHT
or a bit redder because the sun is more like 5000 kelvin
so if something hadnt happened, THE WHOLE SKY WOULD LOOK LIKE THE FACE OF THE SUN

like you copied the face of the sun hundreds of time and pasted it all over the sky until ALL the sky was as bright as the sun when you look at it (a bad idea without dark glass protection)

I'm not sure you can draw the conclusion from the premise. Temperature is dictated by an energy distribution, but brightness is dictated by the number of photons. The sun is 5000K because of the energy distribution of its molecules, but it is bright because of the number of emitted photons.

One question I have, if you happen to know - is the enegy reduction solely from the stretching of space, as it were, or is there an adiabatic expansion also going on (i.e. are the photons doing work on anything?)

[Martin]

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One question I have, if you happen to know - is the enegy reduction solely from the stretching of space, as it were, or is there an adiabatic expansion also going on (i.e. are the photons doing work on anything?)

in standard version cosmology (only kind I know much about) the energy loss is essentially all from enlarging the wavelength i.e. "stretch".
they have not done any significant amount of work (like pushing on matter, exerting light pressure etc.)