The electromagnetic radiation left over from the original formation of the universe in the Big Bang between 10 and 20 billion years ago. It corresponds to an overall background temperature of 2.73 K (−270.4 °C), or 3 °C above absolute zero. In 1992 the US Cosmic Background Explorer (COBE) satellite detected slight ‘ripples’ in the strength of cosmic background radiation that are believed to mark the first stage in the formation of galaxies.

Cosmic background radiation was first detected by US physicists Arno Penzias and Robert Wilson , who in 1978 shared the Nobel Prize for Physics for their discovery. They were using a large horn antenna (from July, 1964 - April, 1965] to map signals from the Milky Way, when they serendipitously discovered the cosmic microwave background radiation. In 2001 NASA launched the Wilkinson Microwave Anisotropy Probe to continue the work of COBE. Further studies will be made by the Planck mission.