All of the below is according to theory, not claiming it to be true. Because DM passes right through itself it doesn't clump like the stuff that made our plane

All of the below is according to theory, not claiming it to be true. Because DM passes right through itself it doesn't clump like the stuff that made our planets do. Instead it remains spread out bound by gravity but not clumping. That makes it incredibly diffuse. But space is also quite big, so it can still contribute a lot of mass.