A faint glow of ancient heat from the early universe still fills all of space.

In 1964, two scientists accidentally found a strange hum on their radio antenna. This hum turned out to be the cosmic microwave background. It is the leftover heat from the Big Bang, cooled down over billions of years. It is like finding warm ashes in a fireplace, proving that a massive fire once burned there. This glow is the oldest thing we can see in the cosmos.