"Relic radiation" refers to faint radiation that was leftover from the early universe. It is often referred to as the "afterglow" of the Big Bang. Full definition
Theory predicts that energetic cosmic rays from farther afield would lose energy before they reached Earth by interacting with relic radiation from the big bang called the cosmic microwave background. (newscientist.com)
Everyday Einstein explains how we determine the age of our universe from ancient stars and relic radiation left over from the big bang (scientificamerican.com)
Space telescopes studying the Big Bang's relic radiation (the cosmic microwave background) have found the universe is almost perfectly flat. (discovermagazine.com)