Many people confuse the colloquial usage of the word "theory" with the scientific usage of the word "theory". Colloquially, a theory is a notion, a conjecture, an hypothesis..., but in science, a theory is a detailed explanation of why something is the way it is. For instance, gravity is a fact (drop a fork to convince yourself). Now, the "theory of gravity" is another matter. In fact, there really isn't a good one. A theory of gravity that explains both macro-scale and quantum phenomena is the Holy Grail of modern physics. But, remember, the lack of a complete "theory of gravity" does not negate the FACT of gravity (drop the fork again if you need to). Evolution is a fact. If you don't believe me, stop taking your antibiotics before you've completed the full course. Or read about the observable (yes, even on a human time scale) changes in the size and shape of finch beaks in the Galapagos in response to climatic variations, or the change in color of moths during the industrial revolution. The "theory of evolution" attempts to explain the workings that lead to the observable, and overwhelmingly documented facts of evolution. The theory is still being debated (e.g. the oft-cited gradualism vs. punctuated equilibria), but the fact that evolution occurs is indisputable. After all, one would never claim that "gravity is just a theory".