Is Our Children Learning Part III: The universe began with a huge explosion.

It's based off of facts. Theories are a coherent group of propositions that explain the facts. Nothing more, nothing less. There is no implication of certainty or uncertainty in that definition. The theories that don't eventually falter to the facts, and in fact make predictions that are testable, are the ones that last.

But it is simply a fallacy to say that direct observation is the only way to know something. For instance, if a woman was pregnant 200 years ago, you could say that she had sex. In terms that we are speaking, that would be the Sex Theory for pregnancy. You may not have actually seen the copulation, but it doesn't mean that the facts don't support your assertion. And just because our explanation is called a theory, doesn't change the fact that she had sex.

Similarly, we can not go back and see the big bang. However, the facts support the assertion.