It's not dark matter. It is MOND.
But anyway this demonstraes my contention that my life is ruled by coincidence because just after posting the dark matter answer and getting off the computer this morning, I sat down to read the new Scientific American.
The cover of SCIAM proposes an atternative theory to explain the effects of dark matter without having to have dark matter.
Well, I did not say that correctly. The alternative is a modification of newton's law F=ma to include a term that makes F proportional to a squared when a is smaller than a naught (a sub 0). A0 is about one angstrom per sec per sec.
The new theory predicts the effects formerly ascribed to dark matter even better than the dark matter theories do.
What really struck me though is that the theory predicts that f~a^^2 for objects that are within the gravitational field of our s0lar system but far beyond Pluto's orbit, beyond 10,000 earth-sun radii. So it seems that MOND (MOdified Newton Dynamics) can explain spacecraft slowdown at these distances. The a^^2 effect is the same as having extra mass in the solar system that is not explained.
I recommend reading SCIAM. It is not on-line, so you will have to come up with 5 bucks or visit you local library.
Fortunately for my first post on this subject, the MOND theory has already been shown to be a subset of dark matter theory in a paper (1) reviewed in the same SCIAM issue.
(1) How Cold Dark Matter Explains Milgram's Law, Kaplinghat and Turner, Astrophysics Journal,569,pg.L19,April,2002
So the bottomline appears to be that MOND is a curve fit to the gravitational effects of dark matter. Thank god- heaven is preserved in physics.