During the Big Bang, a lot of high-energy radiation was created. Then, the universe became bigger and colder. Therefore, the high-energy photons lost most of their original energy. Now, as a result, that radiation is in the microwave part of the electromagnetic spectrum (the microwave part has quite low energy). The cosmic microwave background is the radiation that has been traveling without hitting anything ever since the time the universe became transparent, about 380,000 years after the Big Bang. 