Natural microwave sources
Radio astronomers conduct observations in the microwave region, but due to attenuation by the atmosphere, most of these studies are done using high-altitude balloons or satellites. However, perhaps the most famous observation of extraterrestrial microwaves was conducted by two Bell Labs scientists working on a telecommunications system using a large ground-based horn antenna.
According to NASA's Mission: Science website, "In 1965, using long, L-band microwaves, Arno Penzias and Robert Wilson, scientists at Bell Labs, made an incredible discovery quite by accident: They detected background noise using a special low-noise antenna. The strange thing about the noise was that it was coming from every direction and did not seem to vary in intensity much at all. If this static were from something on our planet, such as radio transmissions from a nearby airport control tower, it would come only from one direction, not everywhere. The Bell Lab scientists soon realized that they had serendipitously discovered the cosmic microwave background radiation. This radiation, which fills the entire universe, is a clue to its beginning, known as the Big Bang."
Penzias and Wilson were awarded the 1978 Nobel Prize in physics for their discovery. The background radiation has since been mapped with great accuracy by satellites. These observations have revealed the minute temperature variations that eventually evolved into the galactic clusters we see today.