The process of natural selection as identified by Darwin is just that, a process. It has since been called 'evolution' as a result of the hypothesis which was developed to explain that process. The hypothesis is called the theory of evolution. It is, perhaps, unfortunate that the word 'evolution' has been applied to the observable process rather than kept just as the label for the theory.
To describe a theory as true goes against scientific method because in scientific terms the word 'true' is an absolute -to say something is true means that it cannot be challenged or denied, such a stance would be unscientific which is why, at best, scientists will only go so far as to acknowledge a provisional truth (which is probably an oxymoron).
We can observe the process of evolution experimentally, we cannot observe the theory of evolution because it is an intellectual construct which has been developed to explain the observable process. It remains possible that another theory could be developed to explain what we currently call evolution in different terms - hard to believe, but possible. We can observe gravity and for a long time we thought we could explain it - recent developments have shown us that may not be the case. Gravity happens but our longstandin