Some people are confused about the meaning of the word "theory" while others are confused about the meaning of the word "brandy".
JLM, in science, we talk about theory of everything. Thus, we have theory of evolution, theory of gravitation, theory of relativity, theory of quantum mechanics, theory of light, theory of sound and so on.
Does that mean that all these concepts are unproven, just pie in the sky speculation? Nothing of the sort. All these theories have substantial evidence supporting them. They are called theories because ultimately, no scientific maxim can be proved, it can only be disproved.
Thus, Theory of Relativity or the Big Bang Theory have been verified time and again by evidence, most scientists, astronomers believe them to be true. However, if tomorrow somebody makes even a single observation that cannot be explained by either of these theories, they are disproved. On the other hand, even if 100 more pieces of evidence are uncovered to support the theories, they still are not proved, just that the probability that they are true increases, approaching 100%, but never becoming 100%.
So no matter how much evidence piles up in their favor, they still remain theories. But that does not mean they are not true, that they are just unfounded speculation on part of somebody.
Scientific theories are based upon solid, proved evidence, and most of them are considered to be true, to be facts.