Even in the absence of light, we do not perceive total darkness. Maybe humans are intrinsically optimistic?
From http://en.wikipedia.org/wiki/Eigengrau, accessed August 16, 2008:
Eigengrau (German: “intrinsic gray”), also called eigenlicht (“intrinsic light”), dark light, or brain gray, is the color seen by the eye in perfect darkness. Even in the absence of light, some action potentials are still sent along the optic nerve, causing the sensation of a uniform dark gray color.
Eigengrau is perceived as lighter than a black object in normal lighting conditions, because contrast is more important to the visual system than absolute brightness. For example, the night sky looks darker than eigengrau because of the contrast provided by the stars.