This is easily the number one creationist argument. They trot it out usually at the very beginning of any creation vs evolution debate, and tend to repeat it ad nauseum. There are three possible rebuttals for this one:
The word "theory" has taken on an unnecessarily perjorative meaning in light of the fact that people do not distinguish between a theory which is supported by the evidence and one is not, ie- an idea which exists "only in theory". A scientific theory is more accurately described as a rational explanation of the facts, and an accepted scientific theory is the only rational explanation of the facts which makes sense. Tell me, would a creationist feel quite as pleased with himself if he triumphantly stated that "evolution is just the only rational explanation of the facts that makes sense"? Of course not; they are taking advantage of the fact that the word "theory" has more than one meaning.
(a slight variation on the previous) Did you notice that the person making this argument simply assumed that "theory" = "unreliable"? Is it impossible to have a reliable theory? If so, shouldn't this person explain why all theories should be assumed to be unreliable? Look at police forensics work; a police forensics expert must invariably construct a theory of what happened in the past, based on physical evidence. Are 100% of police forensics theories bunk? Of course not. And if it is possible to have a reliable theory, then you can't really question a theory by pointing out that it's "just a theory", can you?
Technically, evolution is actually both a theory and a fact, much like gravity. In the case of gravity, the fact of gravity is the observation that objects fall on Earth. The theory of gravity is (among other things) the idea that the force of gravity is proportional to mass, and that all objects create it (yes, even you or I). It has been extensively tested, but of course, nobody was around a million years ago to test it back then; this does not stop anyone from agreeing that there was gravity a million years ago! Similarly, the fact of evolution is that species exhibit both genetic variation and change over generations, and that this corresponds to structural variation and change. This is not in dispute, but most creationists do not even realize that this is considered evolution (they seem to have their own very narrow definition of what it is; how convenient). The theory of evolution argues that this observed mechanism is sufficient to account for the development of life on this planet.
It is difficult to say which rebuttal is the best; that is a judgement call which may depend on the literacy level of your audience. To put it bluntly, some younger or less educated audiences may find the third one too difficult to understand, even though it has the advantage of addressing a serious misconception about evolution which the first and second ones do not.
Fallacy watch:
Non-sequitur (the argument implicitly argues "it's a theory, therefore it's unreliable", which does not logically follow).
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