Questions about is the electromagnetic radiation remnant from the early stage of the universe, also known as "relic radiation".
Cosmic microwave background radiation (short: CMB radiation) is radiation in the microwave part of the electromagnetic spectrum, which comes from all directions in outer space. Since the universe is very large, and the speed of light is constant, we know that when the CMB light arrives from the infant universe, it arrives as the oldest signal that we can detect.
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).