Answer: The correct answer for the blank is- Origin of universe.
Big band theory is an explanation about the origin of universe ( that is how universe began or the birth of universe).
Big Bang theory states that all matter ( including past and present time) in the Universe have come into existence at the same time, that is approximately about 13.8 bya ( billion years ago).
At that particular time, all matter was in the form of very small ball ( that is a compact form) with infinite density and excess heat, which is known as a Singularity.
All of a sudden, the singularity started expanding, and the universe that we know today originated.