Some have asked that question with regard to the very existence of dark matter. MOND offers one alternative, but I've seen observations that contradict it. Plasma Cosmologists propose magnetic fields do the job but that had been well shredded here.

I see no reason to doubt that GR is a very close approximation to the gravitational workings of the universe. This new result with elliptical galaxies is fully consistent with GR. What it is suggesting is that for many elliptical galaxies - no dark matter is needed to explain the rotational dynamics. That doesn't require scrapping GR. It does require that theorists go back and take a look at their CDM models for the universe.

I've pointed out on the baryonic dark matter thread, that some evidence is suggesting the possibility that most of the dark matter in spirals could be cold molecular hydrogen gas. (See that thread for references). If that is the case then GR does not have to be scrapped in favor of MOND or some other alternative to account for the flat rotation curves of many spiral galaxies.

The problem that this result with elliptical galaxies creates is that the CDM models expect all ellipticals to be surrounded by CDM halos. Now we see that is not the case. So it is the CDM models that need a close look.

And the implications could be larger. The current concordance big bang model says the universe is ~23% cold non-baryonic dark matter. If it turns out that most ellipticals lack this CDM and spiral rotational dynamics can be accounted for by baryonic dark matter, then the concordance model would be contradicted and it would be time for Big Bang theorists to assess how in their model it is possible to live in a universe that is in fact dominated by baryonic dark matter. But it probably will take at least another 10 years of observations to establish whether or not the dark matter is mostly baryonic.

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