Einstein disagreed with probabilistic quantum field theory, which is a teachable moment that we all have limits to our capacity to understand.

Dark matter is a factor in multiple scientific models, which in turn are simulations of reality. We only know so much about the real and have to take guesses (often very accurate guesses we routinely risk our lives on). Some of cosmological guesses involve mass with no detectable signatures, id est dark matter.

What is dark matter? We don't yet know. Can something else be otherwise throwing our numbers so that it seems like there's dark matter? Sure.

That's not insanity, but an unknown, dark matter may yet reveal itself (become less dark) or it may go the way of aether and phlogiston.