Seeing as Einstein's theories are based on math + the observable universe (and that they explain events as we see them in our universe), how can we say that our

Seeing as Einstein's theories are based on math + the observable universe (and that they explain events as we see them in our universe), how can we say that our universe is governed by a specific part of Einstein's theory, namely the behavior of spacetime inside of black holes? Seems fairly circular to me.