This was a great question. When you come inside after playing in the sunny outside, why is it so dark?
Simple answer: because your eyes are smart. When you are outside, there is a lot of light. Really, it is too much light. To compensate for this, your pupils (the part of your eye that light goes through) closes some. And then, when you go back inside your pupils are still small. Inside (even with the lights on) is not nearly as bright as outside. Not enough light is getting through your pupils and so everything looks "dark".
Here is a simple demo. While inside, take a flashlight (not too bright - just a plain one) and shine it in someones eye. Watch the eye and you can see the pupil get smaller. I showed this to some kids and they thought it was awesome. Here is a video.
http://www.youtube.com/watch?v=K6ISkIJlClQ