In 1964, physicist Arno Allan Penzias and radio-astronomer Robert Woodrow Wilson noticed an unusual noise always humming away in the background of their radio telescope observations.
The noise – like static on a radio – seemed to be there, no matter what direction they pointed the telescope in the sky. At first, the two believed it was coming from the telescope itself, urban interference, or possibly even pigeons living within the telescope's antenna. However, after removing the pigeons and ruling out all other forms of possible terrestrial interference, the noise remained.
In fact, the noise had been heard by other astronomers, though they had dismissed it as meaningless interference. The reason it could be heard by teams on multiple telescopes and in all different directions was because that irritating noise was in fact the cosmic microwave background (CMB) – the leftover radiation from the Big Bang that is faintly detectable and permeates all of the known universe. Penzias and Wilson realized what they were looking at, and that it was evidence for the Big Bang theory, and won the Nobel Prize in Physics for their work.
Radio telescopes aren't the only way you can access this CMB. If you are old enough to have watched an analog TV, or have an analog radio, you have likely already heard or seen the remnants of the Big Bang. On old TVs when you were between channels (another ghastly phenomenon from the past) you would see a pattern of "snow" on the the screen. Part of this static is from the CMB, though most of it is other forms of interference and (as you'd guess from the name) the CMB peaks in the microwave wavelength.
Modern digital TVs and radios do not pick up and display the CMB, so the young will not know the simple pleasure of turning halfway between channels and watching the leftover radiation from the beginning of the universe while you search for The Price Is Right.