Cosmic Microwave background
The Universe is thought to have begun in the so-called ’Big bang’. While we know very little about the start of the Universe itself, we do have a pretty good idea about what was happening very shortly afterwards. One of the cornerstones of this understanding is the the observation of the Cosmic Microwave Background (CMB), that is, the light left over from the Big Bang itself! The image below shows the CMB, which is basically a map of the temperature we see looking out in all directions from Earth. The fluctuations in this map are actual echoes of the Big Bang itself.
If you want to find out more about the Big Bang and the CMB, check out the Wikipedia entries... here.