In my view, fact is the way the world really is, regardless of whether we understand it or not. For example, the truth about the age and origin of our universe is a fact that we may gradually uncover over time.
A hypothesis is what most people refer to as a "theory" - it's an idea of how something is
A scientific theory is a hypothesis that has been tested and retested and for which there is much evidence, so that it is widely accepted in the scientific community.
A theory can be "proved wrong", but any new hypothesis will need to account for every experiment that has both confirmed and disproved the previous theory. This means that if someone is to disprove the Big Bang Theory, they will have to account for the expansion of the Universe, the Cosmic Microwave Background radiation, and every other piece of evidence that exists.
For example, Newton's theory of gravity started out as a hypothesis, but after thorough checks and research, attempts to disprove it and using the hypothesis to predict positions of planets etc., it became the theory of gravity and remained that way for over 300 years.
Then, it was discovered that the orbit of Mercury didn't quite follow Newton's theory - the point farthest away from the Sun precessed ever so slightly so that the orbit changed over time (due to relativistic effects because of Mercury's orbital speed and position in the Sun's gravity well). Einstein's theories of special and general Relativity "disproved" Newton's laws and replaced our understanding of how gravity and space and time work. They successfully predicted Mercury's motion as well as lots of other observed and confirmed phenomena. They will remain the current theories until some tiny inconsistencies might be observed that need explaining - but they may well be close enough to reality (fact) that they accurately describe the actual Universe good enough for the theory to remain.
There is however a know discrepancy or gap between the Theory of Relativity and Quantum Mechanics that needs to be solved with some sort of theory of Quantum Gravity. Does that mean that the Theory of Relativity is "just a theory" or just a guess? Nope - it just means it does not provide a complete description of reality, but the parts it does describe seem to be accurate and correct.
So a theory is our current best understanding of a subject, while a fact is the way it really is independent of our understanding.