The Microwave background radiation is a remnant of the Big Bang, an ambient background heat residue like the afterglow of a fire. It's temperature is about 2.7 Kelvin, not far above absolute zero, and dropping slowly. It's the average temperature of the universe, or pretty close to it.

In the beginning, the universe was small, and very, very hot. Now, it's big, and much, much, colder. So the temperature has dropped, to so low where it's very hard to find.

In fact, it was first discovered by some Bell Labs people in New Jersey who were working on a microwave communications antenna. They were very dilligent in the jobs, and managed to filter out just about all the static the antenna itself was creating. But they still kept getting a uniform noise of microwave radiation no matter where they pointed the antenna. Kind of like the white static you see on TV while nothing's on. Coincidentally, they had lunch with some researchers from Princeton a few days later, who'd just realized that theoretically, such a microwave background would have to exist if the Big Bang happened. Later, the Bell Labs people won a Nobel prize for this work. (I am told, specifically, that Arno Penzias won the Nobel for this). Of course, how factual this story is is anyone's guess; I heard it from my physics professor, who works in the field.

The COBE satellite produced a very accurate map of the microwave background radiation over all the sky, and detected very small temperature variances in it. This was a good thing, since some sort of irregularity was needed to explain things like galaxies and us. This is because the temperature differences are related to the clumping of matter in the early universe. No clumps, no galaxies.

The microwave background radiation is one of the stronger pieces of supporting evidence for the big bang. When compared to a standard blackbody spectrum, this radiation is more perfectly blackbody than anything we can create in a laboratory. In other words, the Universe is cooling very much according to theory.

More experiments are underway to scan the radiation with more detail. This information can be used for things such as determining whether the universe will keep on expanding, whether it'll recollapse, or whether it's right in the middle.

In other words, the microwave background radiation is cool stuff. A more recent, and more detailed survey of the microwave background was done by the BOOMERanG Project