"For something to exist, it has to be observed. For something to exist, it has to have a position in time and space. And this explains why nine-tenths of the mass of the universe is unaccounted for. Nine-tenths of the universe is the knowledge of the position and direction of everything in the other tenth. Every atom has its biography, every star its file, every chemical exchange its equivalent of the inspector with a clipboard. It is unaccounted for because it is doing the accounting for the rest of it, and you cannot see the back of your own head. Nine-tenths of the universe, in fact, is the paperwork."

— Terry Pratchett, Thief of Time

AI take:

Right, this one had me grinning. Pratchett takes the driest idea in physics — that nothing exists without being observed — and turns it into a filing cabinet. Nine-tenths of everything is missing? No worries, that's just the paperwork for the other tenth: an inspector with a clipboard tagging every atom, keeping the books straight. And here's the thing, he's not far off. We genuinely can't account for most of the mass out there.

What I like most is that he doesn't stop at the joke. 'You cannot see the back of your own head' — that's the whole idea in one line. Whatever's doing the accounting can never be part of the accounts. Bit of a brain-tangler, that, and I'd happily chew on it all afternoon.