It's certainly the case that 'dark matter', like 'Smurf' or 'government regulation', has in recent times become a de facto explanation for the unexplained.   We aren't big believers in magic so mysterious, undetected forces that explain everything probably actually explain nothing - and tossing out Newton in the process  brings on a higher order of scrutiny, since he has been declared irrelevant often before only to survive quite nicely.

Dark matter is currently unable to reconcile all the current discrepancies between measurements and predictions based on theoretical models and competing theories of gravitation have therefore been developed - their problem is that they conflict with Newton's theory of gravitation.

"Maybe Newton was indeed wrong," declares Professor Dr. Pavel Kroupa of Bonn University´s Argelander-Institut für Astronomie (AIfA). "Although his theory does, in fact, describe the everyday effects of gravity on Earth, things we can see and measure, it is conceivable that we have completely failed to comprehend the actual physics underlying the force of gravity." 

Replacing one magical event with another is a problematical solution (see Dark Energy: Is The Theory Of Gravity Wrong?) but two new papers take it on.   Kroupa, Dr. Manuel Metz, Professor Gerhard Hensler and Dr. Christian Theis and Dr. Helmut Jerjen have examined so-called "satellite galaxies", the term used for dwarf galaxy companions of the Milky Way, some of which contain only a few thousand stars.

According to cosmological models, they presumably exist in the hundreds around most of the major galaxies, though only 30 such satellites have been observed around the Milky Way, a discrepancy in numbers which is commonly attributed to the fact that the light emitted from the majority of satellite galaxies is so faint they remain invisible.

Kroupa believes there is more to it.  

"First of all, there is something unusual about their distribution", Kroupa says, "the satellites should be uniformly arranged around their mother galaxy, but this is not what we found."

More precisely, all classical satellites of the Milky Way – the eleven brightest dwarf galaxies – lie more or less in the same plane, like they are forming some sort of a disc in the sky. They also write that most of these satellite galaxies rotate in the same direction around the Milky Way – like the planets revolve around the Sun. 

They believe that this phenomenon can only be explained if the satellites were created a long time ago through collisions between younger galaxies.

"The fragments produced by such an event can form rotating dwarf galaxies", says Metz, recently at the Deutsches Zentrum für Luft- und Raumfahrt (German Aero-space Center).

But there is a catch to this 'crash' theory, "theoretical calculations tell us that the satellites created cannot contain any dark matter.   The stars in the satellites we have observed are moving much faster than predicted by the Gravitational Law. If classical physics holds this can only be attributed to the presence of dark matter," says Metz. 

Yep, if you don't like the magic of dark matter you instead say gravity is wrong.  Certainly gravity has been modified before - when high velocities are involved (through the Special Theory of Relativity), in the proximity of large masses (through the theory of General Relativity), and on sub-atomic scales (through quantum mechanics). - but no one has actually been able to toss it out.

"The only solution would be to reject Newton´s classical theory of gravitation", says Kroupa. "We probably live in a non-Newton universe. If this is true, then our observations could be explained without dark matter."

They contend that the deviations detected in the satellite galaxy data support the hypothesis that in space where extremely weak accelerations predominate, a "modified Newton dynamic" would need to be adopted.

Astrophysicist Bob Sanders from the University of Groningen agrees.  "The authors of this paper make a strong argument. Their result is entirely consistent with the expectations of modified Newtonian dynamics (MOND), but completely opposite to the predictions of the dark matter hypothesis. Rarely is an observational test so definite."


Metz, Manuel; Kroupa, Pavel; Theis, Christian; Hensler, Gerhard; Jerjen, Helmut: Did the Milky Way dwarf satellites enter the halo as a group? (The Astrophysical Journal 2009; doi: 10.1088/0004-637X/697/1/269) 

Metz, Manuel; Kroupa, Pavel; Jerjen, Helmut: Discs of Satellites: the new dwarf spheroidals (Monthly Notices of the Royal Astronomical Society 2009; doi: 10.1111/j.1365-2966.2009.14489.x)