Achluophobia is fear of the dark. Fear of the Dark is the album by Iron Maiden this is not what achluophobia is. We are talking about actual fear here.
Being frightened of the dark is a common component of childhood. Heck, even some of us alleged grown-ups may still fear the dark. Part of that fear is that our imaginations, lacking visual information, can start to make up scary ideas about what might be there.
There is probably an evolutionary reason] for fearing darkness. After all, the night belongs to many predators. Sigmund Freud, on the other hand, suggested that fear of the dark was a manifestation of separation anxiety.
Fear of the dark is reportedly not present in children before the age of two. If the fear becomes so severe that doctors consider it pathological, it gets called scotophobia or lygophobia because big words sound official or something.
If you have a crippling fear of the dark that interferes with your ability to get on with life, please consult a professional who may be able to help you rationalise and control your fear.
Achluophobia is related to Nyctophobia an intense fear of nighttime.
For the author's own fears and overcoming them see: My relationship with my fear of the dark.