When dark refers to night time, darkness or being in the dark, the dream is about your feeling of not knowing where you are going, or how to get to where you want to go with regard to the subject matter of the dream. Other symbols in the dream tell you what you need to do to move out of the dark. For the same reasons dark can also symbolize depression; for example, this would be indicated by dreaming of the upstairs of a house in darkness.
