A faint, diffuse glow coming from all directions in the sky, most intense at a wavelength of around 1 mm; also known as the microwave background radiation or cosmic background radiation. Its existence was predicted from the Big Bang theory, and it was discovered in 1965 by A. A. Penzias and R. W. Wilson. The CMB originated when the Universe was 300 000 years old and consisted of a plasma at a temperature of approximately 3000 K. At this temperature electrons can combine with atomic nuclei, releasing photons which then stream freely across the Universe; this process is known as recombination, and hence this time is termed the recombination epoch. The expansion of the Universe since then has redshifted these CMB photons to an apparent temperature of only about 3 K. Measurements by the Cosmic Background Explorer satellite (COBE), the Wilkinson Microwave Anisotropy Probe (WMAP), and more recently the Planck satellite have shown that the CMB has a black-body spectrum with a temperature of 2.725 K. A characteristic pattern of fluctuations in temperature of only a few millionths of a degree has been measured. The physical scale of these CMB fluctuations is an important standard ruler, providing strong evidence that the Universe is spatially flat. The CMB is regarded as one of the most important pieces of evidence for the Big Bang model.

http://www.esa.int/spaceinimages/Images/2013/04/Planck_CMB_black_background