I believe that some people are afraid of the dark because of what they can't see and also not knowing what is there. It can leave you feeling helpless and is mostly a visual effect. Most scary stories have something to do with the dark, with most bad things happening at night as well as old tales like boogie man which scare people.
Nyctophobia=Fear of Dark Is one of the most common phobias in children. In many cases, childhood nyctophobia passes as the child matures. In older children and adults, however, nyctophobia can become crippling. Nonetheless, most people retain a bit of a fear of the dark throughout life. This fear may be evolutionary in nature, as many predators hunt at night. Consequently, darkness is a frequently used element in horror movies and Halloween events.