Interesting fact: Many people see flying in the air colorless "flies", especially when looking at a bright illuminated surface, such as a clear sky. This effect has a scientific name - the destruction of the glass body. Ideally, the glass body of our eye is a transparent cold matter, but due to diseases, injuries, increased load on the eyes or just age changes, individual fibers in it thicken and lose transparency, which is perceived by us as flies.
p.s.by the way, if the flies appear very sharply, accompanied by flashes of light, this can indicate a rapid detachment of the retina, I recommend going to the doctor, as this will eventually lead to blindness.