So what IS the Cosmic Microwave Background, anyway?
A few hundred thousand years after the Big Bang, things cooled down enough (to about 2,700 ˚C) that neutral matter like hydrogen and helium began to condense from a sea of charged protons and electrons. This released photons that have been propagating through space since that very moment.
Of course, we know that the universe is expanding, right? Those photons are expanding along with it. We are detecting them at a distance in light years almost equal to the age of the universe itself, as they have been stretched and cooled to just above absolute zero (a few degrees Kelvin).
Why “microwave”? The photon wavelengths have expanded so much during the expansion of the universe that they now sit in the microwave range, like extending a Slinky into a single, straight wire!
Check out this cool feature from Space.com to find out even more about the CMB, including how pigeon poop helped us figure out it even existed.