Why some people fear from dark?

From the childhood they tout that in dark area ,there is something like ghost.

Basically, people's fear of the dark is the 'fear of the unkwon'. Alot of us rely on our sense of sight, and when that is denied to us, but our concentration is centered upon it, we tend to let our imaginations run away from us. We think we see something, but are uncertain of it's reality. With uncertainty comes fear. Even when our sense of sight is denied and we concentrate upon sound, identification of such things can easily be miscontrued, again letting imagination run rampant.