I hear this all the time when having discussions about evolution. In one sense it is true that the theory of evolution is, well, a theory. Just as the theory of gravity or germ theory of disease are theories. But there needs to be an understanding about the distinction between a theory and a scientific theory.
In everyday common usage theory is colloquially defined as an informed guess, conjecture, opinion or speculation. You may have all sorts of theories about why Nike shoes make you run faster or why you hit every red light on the way to work or why your lover left you or who really killed Kurt Cobain, etc. Guesswork and contemplative speculation are how we make sense out of the everyday events of our lives. Technically this can be defined as theorizing.
But this is not a scientific theory.
A scientific theory is a well-supported body of interconnected statements that explains observations and can be used to make testable predictions. Scientific theories describe the coherent framework into which observable data fit. The "theory of evolution" is the framework that best explains observed changes of species over time and best predicts the new observations that continue to be made in evolutionary biology and related sciences.
Defined this way, evolution is safely regarded by nearly all scientists as a scientific fact. Most philosophers of science (and I think I may agree with this) claim that one can never know absolute facts. All one can do is lend provisional assent to a "scientific theory" that not only explains current data, but makes predictions and survives falsifiability.
Evolution has done all of this. The "theory of evolution" is every bit as much a fact as the theory of gravitation and the germ theory of disease. People who either ignorantly or dubiously confuse the usage of theory are undermining the rigor with which evolution has been tested and survived in the marketplace. They pretend that scientists are just guessing. Just voicing their opinions. No. All applicable general sciences support evolution with their own independent evidences and observations. Paleontology, biology, chemistry, geology, anthropology, neurology, anatomy, astronomy, physiology, etc.
Let's all work together to properly define these terms. After all, if evolution is untrue then it needs to be met on the firm ground of it's actual claims and not on the wind-blown sand of straw man arguments and misunderstood terms.