So a white hole would be like a bubble or membrane that only allows for one-way passage, that is, light and energy can pass through to the outside but not pass through to the inside?
What would happen when there's no longer any mass or energy inside the bubble? Would the void on the inside combined with CMBR pressure on the outside cause the bubble to shrink into nothingness, which is why we can't detect them?
Any old white holes would have been emptied and crushed to nothiness from the pressure of regular space by now, and so until we can come up with a hypothetical way for them to be created, there's no evidence to suggest new ones are being made any more.