Free Electrons began to join nuclei to form atoms at a temperature below 3,000 Kelvin. When that happened, radiation could pervade through the universe.
The most ancient radiation, emitted 300,000 years after the big bang, represents therefore kind of a snapshot of the universe at the time of recombination era. This radiation is called the Cosmic Background Radiation.
Remnants of that radiation still spread out through the universe. They are observable in the microwave frequency range as a glow dispersed across the entire sky almost uniformly.