The CMB is the oldest known form of radiation and is considered to be evidence for the Big Bang theory of the formation of the Universe.
With a standard optical telescope, the background space between stars and galaxies is almost completely dark. However, a sufficiently sensitive radio telescope can detect the CMB as a faint glow that is not associated with any star, galaxy, or other object.
The CMB was initially composed of extremely high-energy gamma rays. However, as the universe expanded and cooled, these gamma rays have been red-shifted, meaning that their wavelengths have been stretched. Today, the CMB appears as microwave radiation.
The CMB is detected as a faint glow of uniform thermal energy coming from all parts of the sky.
The CMB is a relic of the Big Bang, dating back to about 13.8 billion years ago in look-back time.
The phrase look-back time refers to the time it takes for light to travel from its point of origin to our here-and-now.