Part of the show Naked Science Q&A and Record Breaking Fireworks
Mike in Hawkwell asked:
Where does the microwave background radiation come from, how was it made and why is it still around?
The microwave background radiation is left over from the big bang. Originally the whole universe was so hot it was a plasma and the electrons had so much energy they were wandering about separate from the nuclei. About 300 years after the big bang the universe cooled enough for the electrons to get trapped by nuclei-forming atoms. This released an immense amount of energy in the form of light.