About 76 percent of all non-visually impaired people experience something known as "floaters". These appear as moving structures, like little worms, that sometimes appear in your field of vision if you are staring at something bright and uniform such as the sky, snow, or a white screen.
Their scientific name is Muscae volitantes, or "flying flies" – but they are not insects. As an excellent TED-Ed video explains, they are tiny objects within your eyes. They could be bits of tissue, red blood cells, or protein clumps floating in the vitreous humor. This is the gel-like substance between the lens and the retina that keeps the eye in shape.
Light enters the eye through the lens and activates certain cells on the retina, but as floaters move around the vitreous they cast shadows on the retina, creating the peculiar images that many of us can see.
The video also explains another weird phenomenon of floating moving objects in our vision. It is known, the video explains as the blue field entoptic phenomenon. The video calls this effect the opposite of the floaters. These "blue sky sprites" are not a shadow cast by something in the vitreous humor, but are actually due to white blood cells in the capillaries of the retina.
These immune cells are large enough to slow down red blood cells, creating areas in capillaries containing just plasma followed by the white blood cell and a clump of red blood cells. By looking at a blue bright area, like the sky, we can actually see them moving, as blue light is not absorbed by plasma or white blood cells like it is by red blood cells.