Dark matter has gravity but otherwise does not interact with anything. What could it be? It is localized and is near ordinary matter, mixed in with it, but the Bullet Nebula shows that it can be extracted from the vicinity of ordinary matter, so it is something and does not fill empty space.
Well, which is it? If it has gravity, it must interact.
Couldn't these spacecraft just be running into 'dust' and that's what's slowing them down. Sort of like drag in the air, but at much less magnitude.
I mean for every micro-meteor or particle of dust that strikes the spacecraft, there must be a resulting loss of velocity. Right?
I presume Newtons Laws still apply out there, of course.
L