It terrifies me. Before I started taking medication, I had hallucinations, both auditory and visual). This is all the way back to childhood, and they were always more vivid in the dark. My parents and former roommates tell me that sometimes I'll start to scream in my sleep if someone would happen to shut the lights off (I sleep with the lights on), but since I'm sleeping when it happens, I can't know for sure. I had a sleep study done several years ago, when I was having really horrible nightmares, and they said that I was having night terrors.
In my last apartment, there was the door, then the living room, and then the kitchen. The mud room had a light, but the side of the living room nearest the mud room didn't have light switch to turn on the main light. Apparently the builders thought that the light from the mud room would be enough to walk through and turn on the light from the switch on the other side of the room. Anyway, my point is I'm so freaked out by the dark that I got a permit from my landlord (and another from the city) to have a contractor and an electrician come in and install another switch on the other side of the room, at the cost of $600.
In every apartment I've lived in, I kept a flashlight in every room, except for my bedroom. I kept three in there; one right on the floor by the bed, one in the night table, and one by the door. I keep two in my car, and I have a tritium key fob and a pen light on my keys. Oh, and I drive with the overhead light on. Which, is apparently illegal, and has gotten me pulled over twice.
It's a phobia that you learn to live with.