3 minutes to 380,000 years

The Era of Nuclei marked a pivotal moment in the universe's history, as subatomic particles combined to form hydrogen and helium atoms, making the universe transparent to light for the first time. This transition allowed photons, which had been unable to travel far, to spread across space, creating the cosmic microwave background (CMB) radiation. The CMB, observed by telescopes like COBE and WMAP, reveals tiny temperature variations from the early universe that are believed to have seeded the formation of galaxies. This radiation dates back roughly 400,000 years after the Big Bang and provides crucial evidence for understanding the universe's structure.