And on top of that, every other explaination that people have come up with to explain those data sets has failed.
MOND - the idea that gravity has some extra factor that kicks in on galactic scale has yet to provide even a hypothetical answer to be tested. Last I heard, it gets complicated so fast that they haven't even been able to produce a hypothetical gravitational equation that would explain orbital velocities of galaxies let alone the other observed data.
That the extra matter is out there in more mundane forms such as dim stars or jupiter like orphan planets has been tested since they first started seeing these obeservations, and all tests so far have shown that if that was the case, they should be able top detect it but haven't.
Somebody once made a comment on /. about just because we put out cat food and it is disappearing, it doesn't mean there is a cat in the house that nobody has seen. However, at this point, the cat food is being eaten, the litter box is being used, something is playing with the cat toys, and if you knock on the walls, you can hear something go "meow". It still may not be the case that there is a cat we've never seen living in the house, but if so, the new answer is much stranger and more complicated.