According to the big bang theory, the entire universe began as a
a. supernova explosion.
b. cool, dark expansive nebula.
c. dense, hot, supermassive ball.
d. scattered band of space dust.
The Big Bang Theory is the leading explanation about how the universe began. At its simplest, it talks about the universe as we know it starting with a small singularity, then inflating over the next 13.8 billion years to the cosmos that we know today.
Therefore , the answer is b.