Oh! It's just a theory!
A theory, (as the word is used by scientists) is a detailed description of some facet of the univers'es workings that is based on long-continued observation and, where possible, experiment.
For example we have the description of objects attracting each other according to a fixed rule (the "theory of gravitation"), of energy behaving in discrete bits (the "quantum theory"), of light travelling through a vacuum at a fixed measureable rate (the "theory of relativity"), and so on.
All are theories; all are firmly founded;all are accepted as valid descriptions of this or that aspect of the universe. They are not mere guesses, nor are they wild speculations. And no theory is better founded, more closely examined, more critically argued, and more thoroughly accepted than the theory of evolution. If it is "only" a theory, that is all it has to be.
Creationism, on the other hand, is NOT a theory. There is no evidence, in the scientific sense, that supports it - not one shred. Creationism, or at least the particular variety accepted by many Americans, is an expression of early Middle Eastern legend. It may be fairly described by those who wish to belittle it "as only a myth". Nor is that really belittlement for "only a myth" is exactly what creationism is.