There are theories in which dark matter can be captured, for instance, by neutron stars. This would be very similar to this device. For instance, see here.
What I'm getting at is why does dark matter interact gravitationally?
This is the original reason behind dark matter: we need it to explain the galaxy rotation curves so that it must interact gravitationally. Otherwise, thus would just be other exotic stuff that would probably be called by another name.
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" :)