Why do we sometimes see spherical or cylindrical structures moving in front of our eyes during normal vision?

Image source: taurangaeyespecialists.co.nz

Most of us can see squiggly floating images in our visual field. These floaters are called vitreous floaters and are largely innocuous. The inside of the eye is filled with a jelly like material called vitreous. This jelly like substance tends to undergo liquifaction with age. This process of liquifaction can occur in the early teens or in the late fifties and sixties. When liquifaction of the vitreous occurs a few unliquified particles remain and move around with movement of the eye. Though these particles are usually microscopic in size they are clearly visible to the individual as they are very close to the retina. These vitreous floats are more clearly visible against a clear background, for example, against the sky, a white wall or white paper. Vitreous floaters may increase or decrease slightly with time. These physiological floaters are not a sign of disease. However, sudden appearance off multiple floaters associated with flashes of light or appearance of a very large number of floaters, may be a sign of retinal disorder and should be investigated.

Source: thehindu.com