Gravity and germs, tectonic plates etc are not theories they are established facts, evolution as taught in public schools is a theory, not a proven fact, evolution as a process existing in nature is a proven fact, but the idea that matter moves from a state of supreme primitiveness to more, and more, and more complicated and intricate life forms with out any external influence, just matter creating matter, is by no means possible a proven fact but a theory, that's why they call it the theory of evolution, just like the theory of creationism, or the theory of intelligent design. All three are theories because they are possible explanations for how our world came to be as it is, you may argue one theory is much more likely to be true than another, but you cannot prove 100% that either theory is false.