Creationists argue that evolution is "only a theory and cannot be proven."
As used in science, a theory is an explanation or model based on observation, experimentation, and reasoning, especially one that has been tested and confirmed as a general principle helping to explain and predict natural phenomena.
Any scientific theory must be based on a careful and rational examination of the facts. A clear distinction needs to be made between facts (things which can be observed and/or measured) and theories (explanations which correlate and interpret the facts).
A fact is something that is supported by unmistakeable evidence. For example, the Grand Canyon cuts through layers of different kinds of rock, such as the Coconino sandstone, Hermit shale, and Redwall limestone. These rock layers often contain fossils that are found only in certain layers. Those are the facts.
It is a fact is that fossil skulls have been found that are intermediate in appearance between humans and modern apes. It is a fact that fossils have been found that are clearly intermediate in appearance between dinosaurs and birds.
Facts may be interpreted in different ways by different individuals, but that doesn't change the facts themselves.
Theories may be good, bad, or indifferent. They may be well established by the factual evidence, or they may lack credibility. Before a theory is given any credence in the scientific community, it must be subjected to "peer review." This means that the proposed theory must be published in a legitimate scientific journal in order to provide the opportunity for other scientists to evaluate the relevant factual information and publish their conclusions.
Creationists refuse to subject their "theories" to peer reviews, because they know they don't fit the facts. The creationist mindset is distorted by the concept of "good science" (creationism) vs. "bad science" (anything not in agreement with creationism). Creation "scientists" are biblical fundamentalists who can not accept anything contrary to their sectarian religioius beliefs.