The cosmic microwave background radiation (CMB) is radiation left over from the big bang. When the universe was very young, just as space became transparent to light, electromagnetic energy would have propagated through space at a much shorter wavelength. Nowadays, the temperature of space has fallen to approximately 2.7 K (that’s 2.7 K above absolute zero!) and, using Wien’s Law, we can confirm that the peak wavelength of the electromagnetic radiation is so long that the background radiation lies in the microwave portion of the em spectrum.
The CMB was first detected in 1964 by Richard Woodrow Wilson and Arno Allan Penzias, who worked at Bell Laboratories in the USA. They were building a radio wave detector when they found a source of noise that seemed to come from every direction. That the noise came from every direction ruled out a specific star or galaxy. They were also able to rule out the urban environment but…
“then they found droppings of pigeons nesting in the antenna. They cleaned out the mess and tried removing the birds and discouraging them from roosting, but they kept flying back. “To get rid of them, we finally found the most humane thing was to get a shot gun…and at very close range [we] just killed them instantly. It’s not something I’m happy about, but that seemed like the only way out of our dilemma,” said Penzias. “And so the pigeons left with a smaller bang, but the noise remained, coming from every direction.” source: NPR.org
Here are Wilson and Penzias next to their detector. From the size of the detector, you can see why pigeons might find it a good place to stay.
Wilson and Penzias shared the 1978 Nobel Prize for Physics for their discovery of the CMB. Many of the images we see of the mapped CMB have been provided by a European Space Agency mission called Planck.
Now get some popcorn and watch these videos about the cosmic microwave background.