Floaters occur when the vitreous, a gel-like substance that fills the eye and helps it maintain its shape, slowly shrinks. The vitreous is attached to the retina, a light-sensitive tissue that lines the inside of the eye, and this change in composition causes the vitreous to detach itself from the retina. As the vitreous shrinks, its collagen fibres congregate into clumps and strands. These clumps and strands cast shadows on the retina, and it is these shadows that are seen as floaters.
Although floaters are usually benign and commonly occur with age, there are more serious causes of floaters such as inflammation, infection, hemorrhaging inside of the eye, retinal tears, and injury to the eye.
A comprehensive eye exam with dilation of the pupils is the best way to determine if floaters are benign, or an indication of a more serious problem.