What is cosmic microwave background, and where does it come from?

Cosmic microwave background is a very faint level of microwave radiation filling all of space, and coming from all directions. The cosmic microwave background is useful to physicists as it provides the best available evidence for the Big Bang theory of the universe, and also demonstrates that the universe is expanding.
The Big Bang model states that in the early stages of the universe (roughly 377,000 years after the big bang), space was filled with gamma radiation, the most energetic form of electromagnetic radiation, as a result of an event known as recombination. As the universe expanded, this radiation became less and less energetic, and as the universe aged, this radiation that existed throughout the universe decreased in energy: moving from gamma to x-ray, from x-ray to UV light, from UV to visible light, from visible to infrared and finally to the modern day, microwave radiation.