Perhaps, AgentSmith, a bit like the drunk at night - you're looking for the key under the light because it's the only area you can see. ;-)

If you've been in a room with no windows and switched the light off, then you've 'seen' a totally dark shadow (you're in it). If you've ever looked into a deep cave then you've seen a totally dark shadow.

Not all circumstances provide "radiosity". Dark shadows are more common than you suggest.