The phenomenon that provides evidence for the formation of the first atoms and the subsequent release of photons is known as the cosmic microwave background (CMB) radiation. The CMB is a faint, uniform glow of electromagnetic radiation that permeates the entire observable universe. It is considered the afterglow of the early universe, originating from a time called the recombination epoch, which occurred approximately 377,000 years after the Big Bang.
During the recombination epoch, the electrons and protons in the primordial plasma combined to form neutral hydrogen atoms. This process allowed photons to decouple from matter and travel freely through space. These primordial photons have been stretched and cooled over billions of years due to the expansion of the universe, resulting in their detection as microwave radiation.
The CMB radiation is observed as a nearly isotropic (uniform in all directions) signal with a characteristic blackbody spectrum at a temperature of about 2.7 Kelvin. It serves as a crucial piece of evidence supporting the Big Bang theory and provides insights into the early stages of the universe's evolution.