Heh, the neutron star idea is sort of what I was working up to (I was thinking within jets from similar). So I'm sad at not being original, but happy that someone more well-versed though similarly.
As to the other topic, I'm aware that dark matter was proposed to explain galaxy rotation curves. Is there any idea as to why it interacts gravitationally, or is that still along the lines of 'because it does,b y definition'?
Well, you question may somehow be: how gravity works at the most fundamental level. And the answer is: "we don't know yet, but some are working on it" :)