Everyone has heard about the theory of evolution, but very few people are aware of the scandalous truth about evolution that scientists have kept largely to themselves: evolution is also a fact. Scientists have known this for quite some time, yet for some reason this knowledge has not trickled down to the public consciouss.
This is partly due to a misunderstanding of the meaning of fact. In science, fact means an observation that has been confirmed to such an extent that it is taken to be true. That life descends with modifications is one such observation. At this point in time, the evidence documenting the occurence of evolution is overwhelming.
People also have a misunderstanding of the meaning of theory. In vernacular usage theory is similar to guess, but in science a theory is a falsifiable explanation for some phenomenon (fact) that is well-supported by evidence. Theories are never proven true and never become facts - they can only become better supported by evidence, disconfirmed, or replaced by a rival theory.
The theory of evolution is considered one of the most robust theories in science by virtue of the large amount of consilient evidence from multiple disciplines of inquiry. That the public does not know this is a shame because, as the late Theodosius Dobzhansky once said, "nothing in biology makes sense except in the light of evolution."