cosmic microwave background
The cosmic microwave background is diffuse electromagnetic radiation, most intense around a wavelength of 1 millimeter, that fills the universe. It had been predicted theoretically but was first observed in 1964 by the American telecommunications engineers Arno Penzias and Robert Wilson.
The cosmic microwave background (CMB) is the strongest component of the cosmic background radiation and is believed to have originated in the decoupling era, about 300,000 years after the Big Bang, when radiation was first able to travel freely over great distances without being absorbed by ordinary matter. The temperature of the universe at that time was about 3,000 K, but the expansion of the universe has redshifted the relict radiation into the microwave region of the spectrum so that it now appears as if it has come from a blackbody with a temperature of just 2.73 K.
Sensitive measurements of the microwave background by the Cosmic Background Explorer (COBE) and other spacecraft have shown slight variations in temperature of the CMB with direction; these are taken to indicate slight fluctuations in the density of matter in the early universe, which would have been critical to the formation of the first galaxies.