It’s actually one of the _strong_ points of sciene, not a weakness. Science starts with what we can observe. It tries to find an explanation of what we see. That’s why evolution itself isn’t up for debate: evolution has always been plain to see, Darwin tried, as first scientist, to come up with a plausible hypothesis (in the scientific sense!) on the mechanisms behind it. So Darwin’s theory of evolution isn’t about whether it exists or not, but what it is driven by.