The Big Bang theory was totally rejected at first. But those who supported it had predicted that the ignition of the Big Bang would have left behind a sort of ‘hot flash’ of radiation.

If a big black wood stove produces heat that you can feel, then in a similar manner, the Big Bang should produce its own kind of heat that would echo throughout the universe.

In 1965, without looking for it, two physicists at Bell Labs in New Jersey found it.