The word theory often has been misunderstood, taking it in a sense of something hypothetical, nonetheless in sciences Theory is totally different from a hypothesis. Even more, Theory is above the level of law, why is this? well, the answer is simple, a scientific theory is a set of concepts that includes abstraction of the observable phenomenon, quantifiable proprieties as well as laws/rules, that describe the relationships between the empirical data, observations, and concepts. In other words, Theories include not just the abstraction of the phenomenon but a solid base of previous knowledge and evidence. So theories are built to explain empirical data or observable phenomena and are proposed as a set of principles that explain data.
Taking this into consideration we can see why evolution since Darwin's Origin of species has been called a Theory. It has a solid base of empirical evidence as well as well-supported literature, that has been enriched with the scientific advance, these days we can say that we are working with Evo-Devo, evolutionary developmental theory.