To me, dreaming is only an altered state of consciousness (although it's very similar to waking consciousness). Everything exists inside your head. You don't open yourself to demons or anything else (I believe in Christianity). Besides, with God on your side, what do you have to fear of demons?

It sounds to me like astral projection is dreaming a desired dream. I don't think there is anything to actually spiritually visiting a real place. Just dream places. But, that's just my take.

As for your friend who had the dark thing rush to him or his body, it sounds to me suspiciously like a nightmare. Good luck with your faith and dreams.