Well, darkness is technically all around us. What makes it scary is the unknown, or the fact we tend to see stuff, or that a creature (wolf,bear or cougar) might attack us when we are walking. Maybe after years of constant electrical light, were afraid to coming to the realization that one day we maybe in the dark for years to come. Though not much can be explained for those who are afraid, or maybe it is the realization we might be robbed, or killed, but if you think about it, when you were young you were afraid of a goblin or a boogieman in your closet, when you got older you were afraid of a burglar, maybe some people will forever be terrified of their surroundings in after hours.