Here's why:

* Theory in Science: In science, a theory is not just a guess. It's a well-substantiated explanation of some aspect of the natural world, supported by a large body of evidence. It has been rigorously tested and refined over time.

* Evolutionary Theory: Evolutionary theory explains the diversity of life on Earth through the processes of natural selection, genetic drift, and other mechanisms. It is supported by an overwhelming amount of evidence from multiple disciplines, including paleontology, genetics, biology, and geology.

* Not a Belief System: Evolution is not a matter of faith or belief. It's a scientific explanation based on observable facts and testable predictions.

While some people might use the term "theory" to imply doubt or uncertainty, in science, a theory is actually the highest level of understanding we have about a phenomenon.