When non-biologists talk about biological evolution they often confuse two different aspects of the definition. On the one hand there is the question of whether or not modern organisms have evolved from older ancestral organisms or whether modern species are continuing to change over time.
Biologists consider the existence of biological evolution to be a fact.
It can be demonstrated today and the historical evidence for its occurrence in the past is overwhelming. Well 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.