When something is incontrovertible, it is undeniably, absolutely, 100 percent, completely true. That rain is wet is an incontrovertible fact.
If you look at incontrovertible, you see that -controver-, as in controversy, is hiding inside. Add in the prefix, and it's easy to see that incontrovertible means there is no controversy about something, or, in other words, it is unchangeable and true. Incontrovertible evidence or proof is what you are looking for if you suspect that someone is doing something wrong.