Cosmic Background Radiation
The background of radiation mostly in the frequency range 3x108 to 3x1011Hz discovered in 1964 in a radio telescope as unwanted noise. It is believed to be the cosmologically redshifted radiation released by the Big Bang itself. Also known as the primal glow.
Presently it has an energy density in empty space of about 4x10-14Jm-3.
Once put on a theoretical footing, the finding earned Arno Penzias and Robert Wilson the 1978 Nobel prize.
It is one of the strongest lines of evidence supporting the Big Bang theory.