Many people begin to see floaters as they get older and their eyes age. If floaters have been present before, or if they occur with a diagnosed migraine headache, this condition is usually not serious. They are usually caused by tiny bits of solid tissue that collect in the thick fluid that fills the center of the eye (vitreous gel), blocking light to the retina.
Sudden development of floaters or black dots may mean that a retinal blood vessel has broken and is bleeding into the middle of the eye. This condition, called vitreous hemorrhage, results from the vitreous gel tugging on the retina. It may also be a sign of a serious retinal tear, which requires immediate medical attention.
WebMD Medical Reference from Healthwise