Dreaming of unknown girl is very common and the meaning and interpretation of the dream depends on the situation. If you simply see and unknown girl means there will be progress in life.
Dreaming of you talking to unknown girl means you will have new relationships or friendships. It also means you might stay alone away from home in a distant place.
Dreaming of unknown girl in accident or some trouble means you will face trouble from strangers. It also means you will be forced to help an unknown person and this might bring you better luck.
Dream of unknown girl fighting with you is a sign of problems with opposite sex.
Seeing dream of unknown girl happy with you or playing etc means you will get a desired partner in near future.