system
Given the following post, respond as if you're commenting on Reddit:
human
Why does the cosmic microwave background permeate space instead of just the edges?

When we see light from the CMB, where is it coming from? What actually are we seeing?
gpt
IN THE BEGINNING the universe was formless and void. More precisely, the universe filled with a hot, dense, opaque, and nearly uniform gas. As the universe expanded, the gas became thinner, until it was thin enough that it went through quite a rapid transition and became transparent. The light from that moment of transition was then able to freely travel through the universe. It is still flying around today, because it only stops when it gets absorbed by something.

This light travels at a finite speed, and the universe is either infinite in size, or just plain enormous. Either way, 13.7 billion years after the Big Bang, light is still reaching us from this early transition from opaque to transparent - this light is just coming from billions of light years away. Over the future, we will continue to receive light from the CMB, just from further and further away.

You can flip this around and think of it the other way around if you like. As light travels at a finite speed, the further we look away, the further back in time you are seeing. But eventually you looking far enough back in time that you're seeing when the universe was opaque. This looks like a sort of a solid opaque wall "behind" everything else we see - we can't see anything behind that because the gas is opaque. And this "wall" is the cosmic microwave background.

As for why it's microwaves: the gas was actually hot enough to give off yellow-ish light, but the light has been stretched by the expansion of the universe. It's been stretched out from visible light to longer wavelength microwaves.