Cosmic microwave background radiation (CMB) is a faint glow of microwave radiation that fills the universe. It is a remnant from the early stages of the universe, specifically from about 380,000 years after the Big Bang. At that time, the universe cooled enough for protons and electrons to combine and form neutral hydrogen atoms, allowing light to travel freely. The CMB is nearly uniform in all directions and provides crucial evidence for the Big Bang theory. It has a temperature of about 2.7 Kelvin and is detected using sensitive instruments, helping scientists understand the universe's origin, structure, and evolution.