Cosmic microwave background radiation can be thought of as the echo of the Big Bang. It is the residual energy from the first few hundred-thousand years after the beginning of the universe. According to Universe Today, this radiation fills the entire universe.
The early universe was once much smaller and hotter than it is today, and the extreme heat caused the universe to glow with white-hot hydrogen plasma. As the universe expanded and cooled, that plasma cooled, and eventually the universe became transparent. The residual heat of the Big Bang has since experienced so much redshift that it is now found primarily in the microwave part of the light spectrum.