There seems to be a lot of confusion over what the word "theory" means, and justifiably so. Depending on its context, the word can mean different things.
- A theory in common, colloquial usage means a tentative hypothesis.
- A theory in science means an explanation of how something works, backed by evidence.
- A theory in theoretical physics is a mathematical model describing how something might work.
Hope this clears it up.
(Also, the Big Bang theory isn't theoretical physics; it's astronomy :P)
Theoretical physics theories like the Big Bang Theory and String Theory should really be considered theories rather than hypotheses. While they do seem like they are hypotheses, trying to prove the origins of the universe is a very hard thing to do, so they should be considered theories because they are very hard to actually experiment with.
String and big bang theories are theories for sure because scientists cannot use the scientific method to verify what is true or not regarding the makeup of the early universe. A hypothesis can attempt to be proven using empirical data. Unfortunately, even our best telescopes cannot see the actual big bang to gather information. Until astronomers and cosmologists get a glimpse of the very early universe, the big bang is just a theory that fits the current knowledge we have about the universe.