[apache]

Yeah technically nothing can ever be proven. However, there comes a point at which numerous experiments and calculations, etc. consistantly fail to disprove a theory. At this point it becomes a law, and can readily be taken as fact. Gravity is one of those things. Newton's Laws are others.
However, Einstein's theory of relativity simply cannot be tested at this time. So it will stay a theory until disproven or numerous attempts to disprove it consistantly fail. My point is that it simply cannot even be experimented with, so it is a 'pure' theory, based only on mathematical equations.