Why is the big bang theory call theory?

Answer 1

It is called a theory because that is precisely what it is. It is not called a theory because it 'isn't real'. Evidence is still coming in, and over the next few generations there will be a more solid sense that it is either NOT a good working model of the universe we observe, or it IS a good working model. As is true of any basic theory, it may one day be over-turned and another model will take its place.

Edit: To add to the point made above - Germ Theory is still a "Theory". Although it has not been proven false, nor do we expect it to, so its assumed to be true. Same with gravity, General Relativity is only a theory, but its not been proven false, nor do we expect it to.

That is how science works. Nothing is ever proven true. Theories can only be proven false and when theories stand up to rigorous testing and competition and are not able to be proven false, they then become accepted as being true. Theory of Evolution being a great example. We now know evolution is fact, but the Theory of Evolution is our attempt to describe the process of evolution and the theory.....itself....is still evolving.