Note: If you go blind doing this, it wasn't my fault. However, eye doctors do something like this so I guess it's not that dangerous.
1. Get a huge flashlight and sit in a dark room.
2. Close one eye and over the opposite eye, place one side of the flashlight on the outside of your eye (away from your nose) and move it inward slightly.
3. Enjoy.
I did this by accident a few days ago (don't ask, I don't remember what I was doing) and found it interesting. I guess it's optic nerve endings that you're seeing, but I don't know.