1 {the dark, dull reddish grey fades ever so slightly to an even darker, duller reddish grey}
2 {fading...}
3 {fading...}
4 {fading...}

2021-05-22 Rerun commentary: This sequence, by the way, is pretty much what the early universe went through at one stage. In the early phases after the Big Bang, the pressure and temperature would have been too high for atoms to form. Instead, protons and electrons would have been dissociated and just mixed together as a plasma. (And earlier than that, the constituent quarks that make up the protons would, we presume, have been dissociated as a quark-gluon plasma.)

Plasma, being electrically charged, doesn't allow electromagnetic radiation to pass through easily. So in this stage of the universe, light could not propagate. The entire universe was essentially opaque.

After around 350,000 to 400,000 years by our best estimates, the temperature of the early expanding universe had fallen enough for electrons to start to be bound to protons, forming atoms for the first time. This era is known as recombination, although the term is somewhat misleading in that the subatomic particles are combining into atoms for the first time ever, they're not recombining after having been pulled apart earlier.

Atoms interact with radiation a lot less than plasma does. So light and other electromagnetic waves could now propagate across space, through the universe. The temperature of the universe at this time was about 3000 K. We know this because this is the temperature at which hydrogen ionises and forms a plasma in experiments that we can do here in Earth (or indeed the temperature at which it recombines^[1] as a hot plasma cools down).

So suddenly the universe got flooded with radiation. The radiation was the characteristic radiation of a black body at about 3000 K. At this temperature, any body emits radiation, mostly in the infrared.

Where did this infrared radiation go? Mostly, it didn't go anywhere - it's still around us, everywhere in the universe. However since that time, the universe had expanded considerably, stretching the wavelengths of that radiation. As the wavelengths have stretched, they shifted down the electromagnetic spectrum, from infrared to microwaves. They've stretched close to 1100 times. So the radiation that started as black body radiation with a temperature of 3000 K is now present everywhere in the universe with an apparent temperature of 2.7 K.

This is the cosmic microwave background radiation that we can detect whenever we point a microwave sensitive receiver in any direction in space. That signal is more or less constant and uniform across the whole sky - with some small local fluctuations that are actually quite important in determining just how smooth and uniform the early universe was. The cosmic microwave background radiation is, literally, the first light that could propagate through the universe after the Big Bang, just stretched over time by the subsequent expansion of the universe. Thus it provides evidence of both the Big Bang as an event, and the expansion of the universe, consistent with the physical modelling of how these events unfolded.

You can't see it with your eyes, but it's fairly easy to detect with a microwave receiver. The actual light left over from the Big Bang itself.

[1] Literally, this time.