Floaters in your eye at the edge of visibility are most often the cause of this.
Not to get all Scully on your Moulder. :)
http://www.kellogg.umich.edu/patientcare/conditions/floaters.htmlFloaters: Small specks or clouds moving in your field of vision are called floaters. You may see them more clearly when looking at a plain background, such as a blank wall. Floaters are actually tiny clumps of gel or cells inside the vitreous, the clear jelly-like fluid that fills the inside of your eye. Floaters can have different shapes, such as little dots, circles, lines clouds or cobwebs.
While these objects look like they are in front of your eye, they are actually floating inside. What you see are the shadows they cast on the retina, the nerve layer at the back of the eye that senses light and allows you to see.
When people reach middle age, the vitreous gel may start to thicken or shrink, forming clumps or strands inside the eye. Floaters often occur when the vitreous gel pulls away from the back wall of the eye, causing a posterior vitreous detachment. In some cases the retina can tear if the shrinking vitreous gel pulls away from the wall of the eye. A torn retina is always a serious problem, since it can lead to a retinal detachment.
The appearance of floaters may be alarming, especially if they develop suddenly. You should see an ophthalmologist right away if you suddenly develop new floaters, especially if you are over 45 years of age.