In 1964, Arno Penzias and Robert Wilson decided to use an ultrasensitive microwave antenna to study natural radio emissions from the Milky Way. But what they found instead was an annoying background static. It was there all the time, no matter where they pointed their antenna. They assumed that the problem was with their equipment and spent months trying to eliminate all possible sources of the static, including pigeon droppings inside their giant horn-shaped antenna. Finally they realized that the constant static really must be coming from the sky.
Penzias and Wilson then learned from other astronomers that the microwave background radiation was an expected consequence of the big bang theory. According to the theory, the universe was born in a very hot dense fireball. As it expanded, the fireball cooled. Today, after billions of years of expansion and cooling, the energy of the original fireball remains only as a very faint glow of microwave radiation coming from all directions in the sky.
The discovery of the cosmic microwave background radiation was the first evidence to support the big bang theory, and it convinced most astronomers that the theory was correct. In 1978, Penzias and Wilson shared the Nobel prize in physics for their discovery.