The Armenian Genocide comes to mind. You also couldn't prove that Uyghurs are being subject to anything out-of-the-ordinary in Chinese courts right now.

I'm not saying that that's what we're dealing with here, but saying that "if bad things were happening everyone would already know" is begging the question, not a convincing argument. Noting that we'd expect odd statistical distributions of votes in the case of fraud is a better one.