Cosmic microwave background radiation is also known as relic radiation is an electromagnetic radiation as a remnant of first stage of early universe.
Shortly it is called CMB. CMB is faint cosmic background radiation and a potential source of several data of early stage of universe. CMB is the oldest electromagnetic radiation in universe, During the epoch of recombination.
The space between stars and galaxies is shown as dark by radio telescope. Faint, glow and isotropic background is not associated with stars and galaxies, and more clear in microwave region of radio spectrum.
American radio astronomer Arno Penzius and Robert Wilson in 1964, accidentally discovered Cosmic Background microwave radiation and for their discovery they got Nobel Prize in physics.