In the 1965 Penzias–Wilson experiment, what background radiation filled the universe?

In the 1965 Penzias–Wilson experiment, the cosmic microwave background was detected filling the universe.

Arno Penzias and Robert Wilson were calibrating a horn antenna at Bell Telephone Laboratories when they found persistent microwave noise from every direction. They initially investigated possible instrumental and environmental causes, including material left by pigeons, but the signal remained.

At the same time, a nearby Princeton team led by Robert Dicke was developing a search for the predicted afterglow of a hot early universe. The two groups connected their findings, and the observation was published in 1965.

The cosmic microwave background is relic radiation released when the young universe became transparent, about 380,000 years after the Big Bang. Its nearly uniform temperature is approximately 2.7 kelvins, with tiny fluctuations that later grew into cosmic structure.