Cosmic Microwave Background

Relic thermal radiation from 380,000 years after the Big Bang, permeating the universe at 2.725 K.

Penzias and Wilson found it by accident in 1965 while chasing antenna noise they first blamed on pigeon droppings.
It is uniform to about one part in 100,000, with tiny fluctuations that seeded all later galaxy formation.
Precision measurements of it by the Planck satellite set the universe's age at 13.8 billion years.