I have been having flashes, floaters, and what I can only call "black holes" in my vision for almost a year now. By black holes, I mean something like flashes, but they are black.

I went to the ophthalmologist and he dilated my pupils, and looked in my eyes, He said that my eyes are perfectly fine, no detachments or tears or anything. What else could cause this? Its frustrating as hell. I'm only 20 and it happens more when I'm tired.