Why does everyone have a blind spot?

The answer

Every human eye has a natural blind spot (scotoma) where the optic nerve exits the retina, a region called the optic disc. This area contains no photoreceptor cells (rods or cones) because it is occupied by bundled nerve fibers leaving the eye. The blind spot is approximately 6 degrees tall and 5 degrees wide, located about 12 to 15 degrees temporal to the center of fixation. You do not notice this gap because the brain actively fills in the missing information using surrounding visual context, a process called perceptual filling-in. Additionally, the blind spots of the two eyes do not overlap in binocular vision, so each eye covers the other's gap. The optic disc's location on the retina is an evolutionary quirk of vertebrate eye design; cephalopod eyes (like octopuses) have the optic nerve behind the retina, avoiding a blind spot entirely.