Here's why:

* Theory vs. Hypothesis: In science, a "theory" is a well-substantiated explanation of some aspect of the natural world, supported by a large body of evidence. It's not a guess or a hunch. A hypothesis is a proposed explanation that needs to be tested.

* Evolutionary Theory: The theory of evolution by natural selection explains how life on Earth has changed over vast periods of time. It's backed by a massive amount of evidence from fields like paleontology, genetics, and molecular biology.

* Not a "Fact": While evolution is a very well-supported theory, it's not a "fact" in the sense of an absolute truth. Scientific theories can always be refined or updated as new evidence emerges.

It's important to note that the term "theory" is often misused in casual language. In science, "theory" holds a very high level of respect and confidence, representing a well-established and well-supported explanation.