You have a hole in your vision your brain paints over so you never see it

Where the optic nerve exits each eye there are no light receptors at all, a literal blind spot in your visual field. You never notice it because your brain fills the gap with a plausible guess based on the surrounding pattern, a completion trick you can only catch with a simple two-dot test.

— Edme Mariotte, Blind spot (vision) — First described 1668; standard finding in visual neuroscience