The cosmic microwave background (CMB) is a faint glow of radiation that fills the universe, providing a snapshot of the early cosmos. It originated about 380,000 years after the Big Bang, when the universe cooled enough for atoms to form, allowing light to travel freely. This light has since stretched into microwave wavelengths as the universe expanded. Scientists study the CMB to learn about the universe's origins and structure. By analyzing its tiny temperature fluctuations, researchers can infer details about the distribution of matter and energy, helping to confirm theories like the Big Bang and the existence of dark matter.