One commonly made observation is that evolution is just a theory and that it is not an established fact. It is indeed true that evolution is a theory, but the use of the word theory within that context is a scientific one, and so it is quite different from the common use of the word. Language can be a bit tricky at times and often the way you combine words completely changes what is being described.
In common usage, the word “theory” is used to describe an idea that might, or might not, actually be true. You might ask, “How did X happen?” Somebody might then reply, “Well my theory is that Y caused it”. If you heard that, then you would understand that the answer was simply a guess.
However, the word “theory” has quite different meanings in other contexts. For example if I talk about “music theory”, then this is no longer a vague guess, but is instead describing the examination of the language and notation of music. That is a precise examination of the patterns and structures that make up the fundamental elements of music, there is nothing vague about it. If instead I talked about a “mathematical theory”, I am now describing a very precise form of abstract logical reasoning that consists of a body of definitions, and theorems. So as you can see, the context within which you use a word truly does matter.
In the context of science, a “scientific theory” is defined by the National Academy of Science as …
“a well-substantiated explanation of some aspect of the natural world that can incorporate facts, laws, inferences, and tested hypotheses.”
In other words, when science talks about the theory of evolution, it is not describing a wild crazy idea or a guess, but is instead deploying a term that is used to collectively describe a large body of facts that have not only been well-established through observation, but have also been repeatedly and independently verified. Other examples of the use of the term “theory” like this within a scientific context includes the theory of relativity, atomic theory, or germ theory, none of which are deemed to be “just a theory” by the anti-evolution brigade.
If they are to be truly consistent, then perhaps those who advocate that evolution is “just a theory”, need to consider that if that usage is factually correct, then it would imply that germs are also “just a theory”.