The description of evolution by Darwin is a description of a system. It is philosophy, not science. It is pure coincidence that the strength of similarity between the concept of the system and actual reality is full.
This is better understood if you consider the Relativity Theory by A. Einstein. That was not science, either; it started out as philosophical musings, later some math was slapped on to it to make the system more precisely described, and it turned out that the system matched reality. But that was almost pure coincidence.
Science starts out with observation, and it creates an explanation via the establishing of a background mechanism that can be tweaked to more and more precisely to predict outcomes of future events. For instance, the strength of steel is measured, observed, and later it is predicted and the prediction used, that under certain conditions the same compound of steel will have the same strength.
The "problem" with Darwin's theory of evolution was that he started with observations, and he created an imaginary system that explains reality eerily well, without, in his times, an ability to explain how this was possible. "This is how it works; don't ask me why." He said. We now know why it works that way.
The questionable parts of his theory were, of course, why offspring resemble their parents, and why they differ in some ways from their parents. These two concepts Darwin was at odds of explaining properly, or explaining in the first place altogether.
Add to this concept the confusion created by the religious right (AND the religious left, as you may, plus the religious centre), because their dogma is in direct opposition to the system of evolution, and they prefer to give more credence to their superstitious dogma than to the plausible explanation of observable and consistently happening reality.
-- Updated 2017 June 19th, 5:41 am to add the following --
Londoner wrote:So I would say that 'evolution' is made up of falsifiable theories. But in itself, as a generality, it is the idea that life can be understood through such falsifiable theories i.e through science.
A falsifiable theory in science is a theory which can potentially be contradicted by a physical event. For instance, according to gravitational theory, masses attract each other. It is falsifiable theory, because conceptually one day you may wake up and see things fall up, away from the big mass of Earth.
Evolution is falsifiable inasmuch as a change in DNA structure would not effect a macrobiological change in physiology or body chemistry; or else no change in DNA structure would somehow still produce an offspring totally different from his parents.
This is falsifiable now, because we are in possession of the knowledge of what creates change in diploid offspring; in Darwin's times it was not possible, therefore in his times his theory was not falsifiable, therefore it was not scientific (although bang on.)