If you have a TV that's still capable of receiving analogue signals - or a non-digital radio - you've probably seen the interference that occurs between signals; you hear it as a crackle on the radio, and it appears like a snowstorm on television. The effect is caused by static, and about 1% of that static is left over from the Big Bang - the very birth of the universe.
At the time the Big Bang occurred, the universe was a in a hot, dense state, as you may have heard in the opening music of a TV show with the same name. This heat became what's now known as 'cosmic background radiation'. Because the universe was tiny at the time, the heat had nowhere to go, and has slowly spread and dissipated as the universe has expanded. In every cubic centimeter on space - including everything you can see right now - are around 300 photons of radiation left over from the universe's very beginning, some 13.7 billion years ago.