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This recent paper in Nature (also see here$^1$) claimed to have discovered a galaxy without dark matter. However, in the Standard paradigm of cosmology dark matter played a crucial role in the formation of galaxies. What does this discovery, say about the nature and existence of dark matter? Does it rule out how we explain the galaxy rotation curves?

$^1$The video link that I've cited says that this lack of dark matter in a galaxy strengthens the existence of dark matter though I don't understand why.

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  • $\begingroup$ As any result, it can mean anything and its contrary. People who don't believe in the existence of dark matter will see another refutation, while people who strongly believes in the existence of dark matter are going to complexify their models in order to account for this possibility, by introducing new complicated interaction terms, new free parameters, and other ad-hoc explanations. $\endgroup$ – sure Apr 1 '18 at 12:43
  • $\begingroup$ Since dark and baryonic matter are flowing somewhat independently yet attract, the observation doesn't tell much more than observing a dark matter halo with little baryonic matter inside (which, I seem to recall, has been observed through gravitational lensing). $\endgroup$ – Anders Sandberg Apr 1 '18 at 12:50
  • $\begingroup$ It occurred to me when I heard of this that a bullet-cluster like event early on and involving enough baryonic dust and gas could nucleate a low-dark matter galaxy. Maybe. If the figures work out. $\endgroup$ – dmckee --- ex-moderator kitten Apr 1 '18 at 16:30
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    $\begingroup$ @sure haven't you got this backwards? This observation, if anything, strongly supports Dark Matter, since the alternative explanations involve fundamental properties of nature / gravity, which can't be simply "turned off" for a single gravity, while technically there could be good reasons for the dark matter halo to not be present. $\endgroup$ – Prof. Legolasov Apr 1 '18 at 17:20
  • $\begingroup$ @SolenodonParadoxus: which alternatives? I don't know any good alternatives, but that doesn't mean that this kind of evidences is not supporting your will to dismiss dark matter as a good explanation, thus motiving the search for alternatives. $\endgroup$ – sure Apr 1 '18 at 19:00
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Some physicists believe that dark matter is not a new form of matter, but just an emergent quantum/gravitational effect or due to some modified gravitational dynamics (eg MOND). If that is the case, one would expect similar effects under similar conditions. That is, dark matter should be inevitable in galaxies.

So, not seeing dark matter in a galaxy suggests that its not a result of emergent/effective dynamics, and seems to support the other camp which believes that dark matter IS matter of a new type (some undiscovered particle). That is, those dark particles sometimes did not make it into some galaxies.

Of course we should wait to get confirmation of this latest find, and the detailed analysis. Since very little is known about dark matter, there could be loop holes in any of the above conclusions.

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    $\begingroup$ In MOND, the deviation from Newtonian dynamics expected in a small galaxy, can be suppressed by the external gravitational field of a large neighboring galaxy ("external field effect "). So fans of MOND are waiting to learn more about the neighborhood of this "galaxy without dark matter". $\endgroup$ – Mitchell Porter Apr 1 '18 at 13:36

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