it is a theory accepted by many scientists assuming as real
The Big Bang Theory is the leading explanation about how the universe began. At its simplest, it says the universe as we know it started with a small singularity, then inflated over the next 13.8 billion years to the cosmos that we know today.
Because current instruments don't allow astronomers to peer back at the universe's birth, much of what we understand about the Big Bang Theory comes from mathematical formulas and models.
While the majority of the astronomical community accepts the theory, there are some theorists who have alternative theories— such as eternal inflation or an oscillating universe.