There are many places you can't see when you are a child. These are the places where dark dreams spin themselves like nets. Under the bed, in the closet, in the dark. You only know with certainty the 4 cold inches in front of your nose. It is not(more) just knowing fear, it is knowing you are foolish in contrast to the watcher-in-the-dark.
There are places the mind can go when the eye can't follow. Dreams. Deep space with the uncounted stars, deep water with the boneless fish; other people's hearts.
It is monsters when you are young. When you get older, you are the monster. When you are little you fear teeth, when you are an adult you fear your own decisions. This is why adults lose themselves these days in zombie and vampire shows. We need the reassurance of an external enemy.