Dark matter may have interacted with normal matter in the early universe.  This happened when temperatures in the early universe were extremely hot and both matter and dark matter were in the plasma states.  When the temperature of the universe cooled down an still unknown force could have brought together the the components of matter along with an unknown "stealth particle" to create dark matter.  This dark matter is neutrally charged or else it would be easier to detect. Dark matter is five times more abundant than normal matter yet now interacts weakly with other matter.

http://www.space.com/31013-stealth-dark-matter-universe-missing-mass.html