Vitreous Detachment:

The vast majority of the eye is filled with a gel substance called the vitreous gel.  At birth, the vitreous gel is firmly attached to the delicate tissue lining the back wall of the eye called the retina.  The vitreous gel undergoes changes with age, and small collections of cells or clumps of debris can cast shadows on the retina.  The shadows are experienced as floaters in the vision, and although they seem to be in the outside world, they are actually due to material in the vitreous gel of the eye.  In most people, the vitreous gel eventually releases from its attachments to the retina, called a posterior vitreous detachment.