Haha you're so funny.
Theory doen't mean fact. Theory is an assumption. This is a fact, search it in a dictionairy.
When something is incontenstable you don't call it theory.
Based on the same observations another scientists have other theories. You should read all the theories about the universe
And you should know that 2 people observing the same thing will not see the same or have the same experience. It is relative.
Even reading the same book, people will see different things.
Not all scientist believe in that theory, There is so much others. But if you have FAITH in Big Bang good for you. But it is just Faith.
A fact is something you see, something you can experience, something you can reproduce...
I doubt you experienced or saw Big Bang. Or maybe you do I don't know ))
Even Einstein would be enough humble to admit that he is not sure... that he don't know.
How can you say that big bang is a fact when no one can experience it and no one did and no one will.

But maybe you're all knowing... lol