Cosmic microwave background radiation (CMB) is the thermal radiation filling the observable universe almost uniformly.
Traditionally, the space between galaxies was seen as completely dark through an optical telescope. However, with the invention of the sensitive radio telescope a faint background glow that is not associated with any star, galaxy or other object can be seen. This glow is strongest in the microwave region of the radio spectrum.
CMB is a snapshot of the oldest light in the Universe. It is the radiation left over from an early stage in the development of the universe. CMB was a major landmark test of the Big Bang model of the universe.
When the universe was starting to form it was dense and much hotter. It was filled with a glow from white-hot fog of hydrogen plasma. As the universe started to expand the radiation and plasma grew cooler. The protons and electrons combined to form neutral atoms. These atoms could no longer absorb the thermal radiation and the universe became transparent instead of being an opaque fog.