In cosmology, the cosmic microwave background radiation is a form of electromagnetic radiation discovered in 1965 that fills the entire universe.
It has a thermal 2.725 kelvin black body spectrum which peaks in the microwave range at a frequency of 160.4 GHz, corresponding to a wavelength of 1.9 mm.
Most cosmologists consider this radiation to be the best evidence for the hot big bang model of the universe.
The cosmic microwave background is a prediction of Big Bang theory.
In the theory, the early universe was made up of a hot plasma of photons, electrons and baryons.
The photons were constantly interacting with the plasma through Thomson scattering.
As the universe expanded, adiabatic cooling (of which the cosmological redshift is an on-going symptom) caused the plasma to cool until it became favourable for electrons to combine with protons and form hydrogen atoms.
This happened at around 3,000 K or when the universe was approximately 380,000 years old (z=1088).
At this point, the photons did not scatter off of the now neutral atoms and began to travel freely through space.
This process is called recombination or decoupling (referring to electrons combining with nuclei and to the decoupling of matter and radiation respectively).
The photons have continued cooling ever since; they have now reached 2.725 K and their temperature will continue to drop as long as the universe continues expanding.
Accordingly, the radiation from the sky we measure today comes from a spherical surface, called the surface of last scattering, from which the photons that decoupled from interaction with matter in the early universe, 13.7 billion years ago, are just now reaching observers on Earth.