A "theory" in a whodunit novel is a guess, usually based on flimsy or circumstantial evidence to enhance the plot and keep the reader deeply engrossed.<quoted text>I understand quite well what you mean.Scientific theory is just an educated guess.While a fact is certain.
A scientific theory is "a well-substantiated explanation of some aspect of the natural world, based on a body of facts that have been repeatedly confirmed through observation and experiment." (National Academy of Sciences)
That biology on this planet changes over time is a well established fact. We call this change over time "biological evolution". We also have a well tested model explaining how these changes in biology occur, and this is called the Theory of Biological Evolution. Therefore the general term "evolution" IS both a theory and a fact, dependent on what you are referring to. This is well covered in basic middle school biology class.