Astronauts can see stars with their eyes closed!

Starting with Apollo 11, astronauts have reported that they saw frequent flashes of light, even when their eyes were closed. This was surprising, given that their cabin had low light conditions and was shielded from the outside.

The reason for this is that there are particles called "Cosmic Rays," that travel freely in space, but not get stopped by Earth's atmosphere. They're so small that they pass through eyelids easily and produce this effect. Scientists have 2 possible explanations for this phenomenon: One think that these particles (which are charged) cause the eye's vitreous humor to produce light.   The other is that the flashes are caused by the cosmic rays hitting the retina directly.