Remember when you were scared of the dark as a kid? I bet your mother always told you “There's nothing in the dark that isn't around during the day” or something similar. As a child, you eventually believed them, and now sleep peacefully, I would assume. However, their logic is terribly, and possibly fatally flawed.
Every child naturally fears the dark, or that's what parents are led to believe. In reality, they fear what hides within its shadowy confines; and are wise to do so. Evil has always done its most terrible acts under the wings of night. Have you ever wondered why?
Evil entities are tied to the darkness at a fundamental level, some to the point where they become darkness incarnate. They are at home beneath their Cimmerian veils, and thus shun the light.
Why do they avoid the light, you ask? Light, as is its nature, reveals the truth about all things. Evil beings who glance upon their true selves often go mad on the spot. By the same token, darkness obfuscates the senses, and makes it easier to catch one's quarry unawares.
Many are the demonic entities that can blend seamlessly with the dark places, and are thus almost imperceptible to humans. These terrible beings have tormented mankind since the dawn of time, and thus our hereditary aversion to the lightless places is explained.
Remember this when you are in an ill-lighted area on your own, for the creatures of shadow are always watching, and hungry. So, the next time you are told not to fear the gloom; tell the other person of the true, fell nature of the dark!