That life on Earth during the past 3.5 billion years has evolved from single-celled organisms to complex and diverse creatures, including humans, is a fact. This fact is based on the recorded and published observations of thousands of scientist over the past 200+ years. It is evidence that is clearly written in the geological and biological record.  The observation that evolution has occurred is now as clear and undeniable as the Earth circling the sun, or that matter is made of atoms, or that the Earth’s continents move. 

The only theory that adequately explains how evolution happens is Darwin’s theory of natural selection (descent with natural modification), or usually referred to as “Darwin’s theory of evolution”. This is what we are talking about (or should be talking about) when we say “the theory of evolution.” There have been some modifications and some minor details are still being investigated, but it now stands stronger than ever. The “theory” that evolution occurs through the process of natural selection is so overwhelmingly documented that it is as close to a fact as you can get in science. Nothing makes sense in biology except in the light of evolution. Regardless of what fundamentalists have been saying for hundreds of years, every day that passes brings more observations confirming Darwin’s theory of how evolution occurs. Thus, one can just as confidently say that it is also a “fact” that evolution occurs through the process of natural selection.

The word “theory does not belong in front of the word “evolution”.  We don’t speak about the “theory of the Earth circling the sun”, or the “theory of atoms”, or the “theory of the continental plates”.  The “theory” comes into the discussion when we try to explain why the Earth circles the sun, or why atoms exist in the configurations they do.  The word “theory” does not belong in front of the singular word “evolution” except in the context of  “Darwin’s theory of evolution”.  Evolution of life on Earth stands as a fully documented observation, as a fact.