The fact of evolution is the allele frequency of a population changes over time. This is an observable phenomenon. It's a fact. The theory of evolution gives us explanations to how/why the allele frequency of the population changes over time.
Someone brought up economics a couple posts before this. It's a very interesting analogy. Do you doubt that the economy exists? It's an observable fact the economy exists. Then we have the theory of economics.
Same thing with disease. Do you doubt diseases exist? It is an observable fact that diseases exist! Then we have the germ theory of disease.