Eventually someone has to LOOK and see what they see, and take it as a given self evident truth that if they see something it must exist and be true. Nirvana is the same way. It is not some mechanical theoretical out thereness which you can never see and so can only hypothesize and get peer review about. Nirvana is inside you, it is part of your own consciousness. It IS what you finally look at and say, yes I see it, therefore it is true.
All nirvana is, is something to see.
Consciousness is self luminous, you don't need something ELSE, some other evidence, to support to yourself that you see. It is not a theory that you see. Not only does Nirvana exist, but it also has a wonderful sense of humor.