What is the remnant of the radiation that filled the universe?

The remnant of the radiation that filled the universe is known as the Cosmic Microwave Background (CMB) radiation. This faint glow of microwave radiation is a relic from the early universe, specifically from about 380,000 years after the Big Bang, when the universe cooled enough for atoms to form and light to travel freely. The CMB is remarkably uniform but has slight fluctuations that provide valuable information about the universe's structure and evolution. It serves as a crucial piece of evidence for the Big Bang theory.