Your eyeball is filled with a clear, jelly-like substance called vitreous humor, which helps the eye maintain its shape. Inside the vitreous humor, dead cells and stringy bits of protein cast tiny shadows on the retina, shadows that are responsible for the floaters you see in your field of view.
When you notice a floater, you may shift your gaze to look at it, only to find it seems to jump away from your glance. Floaters stay roughly in the same place in front of your retina, so moving your eye to look at them causes the floaters to appear to move, too.
People of all ages can have floaters, but they become more common as we get older. Some floaters are stable for many years, while others come and go over a period of weeks or months. Floaters are usually easy to ignore and thus don’t present a problem for most people. However, a sudden dramatic increase in floaters can be an indication of a retinal detachment and should be checked by an eye doctor.
Despite the name, many floaters tend to sink to the bottom of the eye. If you happen to be lying on your back, this tendency puts the floaters directly in your line of sight as you gaze up to the sky—an optimal arrangement for viewing.