It is a theory in that they are far from understanding the mechanisms by which evolution has occurred. In the eloquent words of evolutionist Stephen J. Gould, Evolution is a theory. It is also a fact. And facts and theories are different things, not rungs in a hierarchy of increasing certainty. Facts are the world's data.
Theories are structures of ideas which explain and interpret facts.
Facts do not go away while scientists debate rival theories for explaining them. Einstein's theory of gravitation replaced Newton's, but apples did not suspend themselves in mid-air pending the outcome. And human beings evolved from apelike ancestors whether they did so by Darwin's proposed mechanism or by some other, yet to be discovered. There are numerous problems with this explanation. First, if evolution is a fact, then evolution is equivalent to data.