Close your left eye. Stare at the cross below with your right. Somewhere to its right, there is a place where a dot simply is not there. No flicker, no dark patch: the dot is gone, and the world looks perfectly whole. That gap is real, it is in both of your eyes right now, and your brain has been painting over it your entire life.
Close your LEFT eye (for the right-eye setting). Look only at the cross. Drag the dot sideways, or use the arrows, and move your head slowly closer or further until the dot vanishes. On a phone, hold it close.
The back of each eye is lined with the retina, a sheet of light-sensing cells called rods and cones. Each of them is wired to a nerve fibre, and all those fibres have to get out of the eye somehow. They bundle together and leave through one small round exit, the optic disc, where they form the optic nerve and run to the brain.
That exit is crowded with nerve fibres and blood vessels, and it has no rods or cones at all. Light that lands on it is not detected. So each eye has a patch of the visual field it cannot see. It sits about 12 to 15 degrees to the outer side of where you are looking, a little below the horizontal, and it spans roughly 5.5 degrees across and 7.5 degrees high.
Edme Mariotte, a French physicist, described it in the 1660s. His trick was the one you just did: fix one eye on a mark and watch a second mark disappear.
Angles are what the eye cares about, but a screen or a wall is measured in centimetres, so the gap between your cross and the vanishing point depends on how far away you sit. With the angle fixed at about 15 degrees:
separation ≈ distance × tan(15°) ≈ 0.27 × distance
Sit 50 cm away and the dot has to be about 13 cm from the cross. The patch itself is about a tenth of your distance wide: a bit under 5 cm at 50 cm. Slide the distance in the lab above and watch the numbers. It also tells you why the trick fails when you are too close or too far: the dot lands outside the patch.
You do not notice a gap in your vision for three reasons. First, the two eyes cover for each other: the hole is on the outer side of each eye, so the other eye usually sees that part of the world. Second, your eyes keep moving, so any one spot is only briefly in the blind patch.
The third reason is the strange one. Even with one eye shut and still, you do not see a hole. You see whatever surrounds it, continued across. Try the stripes in the lab: the dot vanishes and the stripes run straight through where it was. Try the grid of dots: the pattern closes up. The brain fills in what is missing from its surroundings, and it does not tell you it did.
This filling-in is not just a guess about plain colour. In a 1991 study, V. S. Ramachandran and Richard Gregory showed that people fill in textures, not only uniform colour, when a patch of the screen is surrounded by moving noise. Your brain's job is not to show you light as it lands. It is to build the most likely scene.
Vertebrate retinas are wired as if backwards: the nerve fibres run across the front of the retina, the side facing the light, and then dive through it at the optic disc. Octopus and squid eyes evolved with the fibres behind the light-sensing cells, so they have no such exit hole in their field of view.
Your eye gets away with it because filling-in makes the flaw almost free. The next time you look at a plain wall, remember that a patch of it is a story your brain is telling you.