Ciro Santilli Surely You're Joking, Mr. Feynman chapter An Offer You Must Refuse (a play on words on The Godfather (1972)) has an interesting historical mention from the early 1950s while at Caltech:
The next day, I had the greatest luck in making a decision. God must have set it up to help me decide. I was walking to my office, and a guy came running up to me and said, "Hey, Feynman! Did you hear what happened? Baade found that there are two different populations of stars! All the measurements we had been making of the distances to the galaxies had been based on Cephid variables of one type, but there's another type, so the universe is twice, or three, or even four times as old as we thought!"I knew the problem. In those days, the earth appeared to be older than the universe. The earth was four and a half billion, and the universe was only a couple, or three billion years old. It was a great puzzle. And this discovery resolved all that: The universe was now demonstrably older than was previously thought. And I got this information right away - the guy came running up to me to tell me all this.
The existence of this is quite mind blowing.
It is basically perfectly black-body radiation, with a very faintly by measurable anisotropy (slightly less or slightly more some regions) due to quantum fluctuations of the early universe.
To make sense of it, you can think of the universe as the expanding raisin bread model, but it expands faster than light (thus the existence of the cosmological event horizon), so we are still receiving light form the middle, not the borders.
The ansiotropies of CMB are likely the best astronomical compass we will ever have, as it is the thing with the least proper motion.
