"Drifters"

[Johnny Pneumatic]

What causes "drifters"(the things that look like tiny bubbles chained together and some look like bacteria. I see them when I look at the blue sky)?

[Wrath of the Swarm]

They're fragments of blood vessel broken off inside your eye. They're normally called 'floaters'.

[Reginald]

From here http://www.allaboutvision.com/conditions/spotsfloats.htm

For most people, floaters occur as they grow older. The vitreous humor thickens and clumps as we age, and floaters result from the clumped vitreous gel. Sometimes pregnant women see spots caused by little bits of protein trapped within the eye. Eye injury or breakdown of the vitreous humor may also cause spots and floaters.