What you are seeing when you stare off into space is bits of retina. Imagine your eyeball is a solid ball, with a small clear patch to let in light in the front (your pupil). The light travels through perfectly clear liquid that completely fills the eyeball, and to the lining of the eyeball, the retina, which picks up on this light and sends a message to your brain so that you see things. Now, the retina is made up of millions of nerve cells. However, sometimes little tiny bits of the retina break off, when you bump your head or something. You don’t lose enough to go a bit blind, or anything, though. So these broken bits of retina float around in the liquid in your eyeball.

When you stare off into space at something really bright, like the sky on a sunny day, you have a lot of light entering into your eye but nothing to focus on. Those broken bits of retina in the liquid make a shadow, so that they are perceived as “dark” by your brain. The broken bits are irregular are look like little bacteria or worms.

You don’t see them when you look at something in particular (like watching TV) because your brain is intent on focusing on and interpreting the specific patterns of light from the TV, and ignores the miniscule shadows from the broken bits floating in your eyes.

(Source: 10uhclock)