I know this because I have suffered from this phobia for most of my life, as to why I have this fear - well, it is hard for me to say what caused it but I have a theory. Granted my counsellor has told me many times that my mind can play tricks on me, and as much as I try, I can't block out what I believe are memories of an event which could very well explain my phobia.
It began when I was roughly six years old, I was sleeping quite peacefully in my room when I suddenly heard a bang that made me wake with a start - I couldn't see anything but pitch darkness and as I cried out I felt something heavy land on my bed and begin to press against me, as if something was laying down on top of me.
I cried out more and began to struggle as this mass pinned me down and I could feel the sound of raspy breathing above me, the smell of rotten eggs filling the air - yet I couldn't see anything as the darkness remained.
I felt a clammy hand reach over and clamp against my mouth, my cries muffling as I panicked - yet in a moment the experience ended as my parents entered the room and switched the light on: revealing nothing.
My parents tried to convince me it was a bad dream, yet I was too scared to sleep alone that night and thus I spent the remainder of the night safe in my parent's room.
From then on I flat out refused to sleep unless I had a light on, my parents tried many tricks to snap me out of it and my dad (bless his heart) even slept on the floor some nights to try and comfort me: sadly no matter how often they did this my fear simply would not fade.
Even to this day I refuse to sleep without a light on and this has caused more than a few chuckles amongst friends, who don't understand the fear I experience - my counsellor continues to tell me that the best way to end the phobia would be to expose myself to darkness, to learn there was nothing to be afraid of.
Yet as much as they mock me, as much as my counsellor tries to reason I simply won't do it - I can't face the dark.. because this is not a fear of darkness.. this is a fear of what may wait for me within: a childhood bogeyman who may just be waiting for a chance to finish whatever it had planned all those years ago.