Astronomers believe that cosmic background radiation formed:

a. about 1 billion years ago.

b. about 1 million years ago.

c. shortly after the big bang.

d. about 100,000 years ago.

The assumption that the cosmic microwave background

is the remnant of the environment at the time of the big bang

or very shortly after it explains very well what we observe now.

So it is the current prevailing theory. "Believe" is not involved.