Space is malleable, and has a certain 'springiness'. I think the effects we see on gravitational pull by 'dark matter' is actually 'tired' space. A long time ago, a very large amount of energy caused certain sections of space to become stretched too far, like a balloon that was filled with air, then having all the air let out (ie space is not as uniform as we think).
When matter is travelling through this 'tired' space, gravity is not behaving the same way because the space that the matter travels through is worn and 'tired' and affects the velocity of the matter.
These pockets grow as the universe expands.