Cosmic microwave background radiation
Also called relic radiation or CMB, it is diffuse cosmological radiation at 3 K.
This ambient radiation comes from all directions in space, and its properties are those of a thermally radiating black body at 3 degrees kelvin. It was discovered by A. Penzias and R. Wilson in 1965 and is explained as being the relic of a period of intense heat that the universe went through after the Big Bang about 13 billion years ago.
The black body spectrum of the CMB obtained with COBE.
Cosmic microwave background radiation - 1 Photo