While we don't know everything, what we don't know must fit without the things we already know. It seems highly unlikely there is something significant in this area we don't know.
Not belief, things we have exponentially observed. Either you need to explain why the observation was actually incorrect. Or you need to have a theory that explains the observation. We believe relativity is correct because we have lots of experiments and observations that show that it's correct. That doesn't mean relativity is actually correct. However, it means that if it's wrong, whatever actually is correct must somehow encompass what relativity predicts in the same way relativity currently does.