I didn't read that positive matter is attracting negative matter. What I gathered is that the presence of a cloud of negative matter in the right place would be

I didn't read that positive matter is attracting negative matter. What I gathered is that the presence of a cloud of negative matter in the right place would be held in place by mutual repulsion of the outer arms of the galaxy and the inner galaxy. And that the neg matter supplies enough repulsion to the diffuse outer galaxy that it can spin a bit slower than it would have to otherwise.