The Cosmic Microwave Background was created as the first atoms formed hundreds of thousands of years after the Big Bang, and it retains information about the formation of the Universe. Discovering it existed confirmed a key prediction of the Big Bang Model and won its discoverers a Nobel Prize. Another Nobel went to the team behind the Cosmic Background Explorer, which gave us our first look at some of the details of the Cosmic Microwave Background (CMB), providing support for the idea that the Universe underwent a period of inflation.
Source: The mission to learn everything we always wanted to know about the Universe