Object permanence is technically an axiom. The idea that things exist even when we aren't observing them.
There's also a problem with terms, particularly related to quantum mechanics. It uses the term observer. To a layman, that's a person watching. To a scientist its any collection of atoms/fundamental particles that can cause the quantum waveform to collapse.
The results of the axiom are that things do exist when we are not observing them. Our observations don't back propagate to retroactively bring them into existence. We can't prove that however, though it's fundamental to a lot of science making sense (quantum mechanics being the oddball).