This is part of an answer by Jon Groubert on Quora:
[…] the science of cosmology was practically born when two guys, Arno Penzias and Robert Wilson, who were working at Bell Labs were trying to find the source of a persistent hum in the background of telephone lines. Bell had tried everything to get rid of this hum, all sorts of experiments, nothing - it was still there.
In 1964, these two guys were using a type of radio telescope as a detector to see if they could point to something in the sky as being the source.
This is the Holmdel Horn Antenna in Holmdel, NJ. There they are at the bottom of the antenna. But no matter what they pointed at in the sky, they always found the same hum, coming from every direction. Maybe there was something wrong with the antenna? So they went inside the antenna, and found a pigeon nest and pigeon droppings in there. They cleaned it out, killed all the pigeons. The hum was still there, no matter where they looked.
They hypothesized that what they were observing, this hum, was the leftover radiation from the Big Bang. And they were right!
