Could be a physical defect in the top layer of the eye, or cataracts/something in the “jelly.” You move your eye, but certain parts don’t necessarily move with it and any scrape, scratch, or object in the eye in those areas will “float” across your vision like that.

Or you’re describing what happens when you look at a bright light long enough to get “floaters.” That’s just from temporarily burning out the receptors in your eye so it becomes darker in those spots. It’s also why you shouldn’t stare directly at bright lights; too long and that shit becomes permanent.