Untestable theories aren't science. The theory of evolution is an example. It is too broad to test conclusively, so therefore it isn't science, just speculative philosophy, and those who teach it aren't scientists, in my view.
Certainly, some observations may seem to support the theory, and others negate it, but there is no test that can be conducted that can conclude all (or any) species of plant or animal experienced Darwinian evolution to get to where they are today. There are just too many variables and unknowns to reach such a conclusion.
All we can do is speculate and that isn't science, which makes it strange that so many people think Darwin's theory of evolution is an important scientific theory. It isn't, because it's untestable and therefore impossible to prove or disprove. This makes it outside the domain of science, which can only speak authoritatively about things it can conclusively observe and test for, but places it within the domain of philosophy, which is full of such speculative theories.
See also: The only evolution that matters