Ed Pearlstein
"Only a theory" is what creationists like to say about evolution. And that seems to carry greeat weight with some people.

Such people don't understand what that word means to a scientist.

As used in science, "theory" does not mean the same thing as it does in everyday life. A theory is not a guess, hunch, hypothesis, or speculation. It is much more full-blown.

A theory is built upon one or more hypotheses, and upon evidence. The word "built" is essential, for a theory contains reasoning and logical connections based on the hypotheses and evidence. Thus we have Newton's theory of gravity and the motion of planets, Einstein's theory of relativity, the germ theory of disease, the cell theory of organisms, plate tectonics (theory of the motion of land masses), the valence theory of chemical compounds, and theories of evolution in biology, geology, and astronomy. These theories are self-consistent and consistent with one another.

Construction of good theories is a major goal of science.

Yes, a scientific theory can be wrong, as shown by experiment or observation, since one of its hypotheses might be wrong or the reasoning might be flawed or new data might come along that disagree with it. Or its validity might be limited (as are some of those listed above). So in science, a wrong theory gets modified, discarded, or replaced. This has happened, for example, in physics with the caloric theory of heat and the theory of the luminiferous ether, and in chemistry with the phlogiston theory of combustion.

In physics, which is my field, theories such as classical mechanics, thermodynamics, and electromagnetism are thought to be on excellent ground in both evidence and reasoning, but each of them is still "just a theory". Other theories, such as in cosmology and elementary particles, are still being developed, and do get changed as new evidence and reasoning come in. The fact that theories are subject to improvement is the great strength of science.

Supernatural creation is not a theory, but a hypothesis. Considered in a scientific sense, it has a fatal flaw: it is sterile. If someone asserts that there is a creator-god, one can ask "So what?" Nothing follows from it; it leads nowhere. Some religions have additional hypotheses, such as: only one creator-god, a great flood, the sun standing still, a virgin birth, a trinity, a resurrection, the efficacy of prayer; but no one of these is logically demanded, or even suggested, by the others. They are just added on.

Anti-evolutionists sometimes say that evolution has not been "proven". In a strict sense, no theory is ever proven in any field, with the possible exception of pure mathematics, since new data might come along that require a change, and there are always details that haven't been tested. Sure, there are things not yet understood about evolution, as in many other fields; but that is why scientists do research! I have encountered the statement - meant as a put-down - that scientists don't know everything. Well of course not, but we expect to know tomorrow more than we know today.