Evolution is a fact. It has been observed many times (pdf), and the evidence in the fossil record for it happening in the past is overwhelming. This is the same as gravity is a fact: you drop something, it falls — every time.
The full title of Darwin’s book is On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life. It’s important to keep things straight. The existence of evolution is not a theory, it is an observed fact. Why and how it happens is open to debate.
All of this is not to say that opponents of the “theory of evolution” have proposed any reasonable alternative or objection. Their arguments are like tissue paper. But at least we should be able to get the terms of the disagreement straight.