The universe's oldest light echoes the Big Bang

The cosmic microwave background (CMB) is the universe's oldest light, a faint glow of microwaves that fills all of space. This residual heat from the Big Bang, discovered accidentally in 1965, is crucial evidence for how our universe began. It's like a cooled-down afterglow from when the universe was just 380,000 years old, about 13.8 billion years ago.