Question

which best describes cosmic microwave background radiation?
radiation thought to exist but not yet measured
radiation predicted to exist in millions of years
radiation that existed before the big bang
leftover radiation from the big bang

Explanation:

Cosmic microwave background radiation is the after - glow or leftover radiation from the Big Bang. It has been measured and exists now, not something predicted for the future or from before the Big Bang.

Answer:

leftover radiation from the big bang