Cosmic Background Radiation, Redshift and the Expanding Universe.

At an earlier time, the universe was filled with hot opaque gas. As the universe grew colder with this expansion, the gas went from ionized atoms and electrons to neutral atoms and the gas became transparent. All the light that had been continually emitted, absorbed and scattered was now able to fly freely as if every point in the universe simultaneously emitted thermal radiation at the temperature of the gas. As the universe continued to expand, that radiation was continuously redshifted and we see it now as microwaves with an effective temperature of about 3 Kelvin.