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When I first heard about star velocity as a function of its distance from the center of the galaxy and the difference between prediction and observation I immediately thought "there must be a threshold where gravity reacts differently". Then I discovered MOND theory and that it can't explain gravitational lensing.

But what if observed gravitational lens is only well placed black holes ? After all we detect black holes with their gravitational influence.

So, why dark matter instead of MOND + more black holes ?

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You should not take claims about the failure of MOND to reproduce strong gravitational lensing too seriously. In this review article the authors, Benoît Famaey and Stacy S. McGaugh, state:

Due to the fact that all the above models were using the Bekenstein μ-function (α = 0 in Eq. 46), and that this function has a tendency of slightly underpredicting stellar mass-to-light ratios in galaxy rotation curve fits, it was claimed that this was a sign for a MOND missing mass problem in galaxy lenses. While such a missing mass is indeed possible, and even corroborated by some dynamical studies of galaxies residing inside clusters (i.e., the small-scale equivalent of the problem of MOND in clusters), for isolated systems with well-constrained stellar mass-to-light ratio, the use of the simple μ-function (α = 1 in Eq. 46) has, on the contrary, been shown to yield perfectly acceptable fits in accordance with the lensing fundamental plane.

(I have suppressed the references in the above paragraph).

Thus the claim that MOND has problems with gravitational lenses is wrong (in my opinion), or at worst disputed.

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  • $\begingroup$ Ok, so what is wrong with MOND ? I hear and read about dark matter everywhere but not MOND. $\endgroup$ – xxxo Aug 14 '14 at 11:33
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    $\begingroup$ @xxxo 1. the theory is ugly, especially in its general relativistic form. 2. It is said in the paragraph I quoted above: some isolated systems, and certaily clusters of galaxies, still need dark matter even if you use MOND rather than conventional gravity. In other words: MOND seems unable to explain away all dark matter, especially in systems with lots of observations to be explained. $\endgroup$ – MariusMatutiae Aug 14 '14 at 11:39
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    $\begingroup$ @xxxo: MOND simply violates Occam's razor more than cold dark matter does. It introduces a completely new physical dynamic without solving the problem (the hypothesis just hides the problem behind one or several parameters which remain completely unexplained). In comparison, cold dark matter doesn't assume anything that we haven't seen before: a weakly interacting particle that is hard to detect directly. Cue neutrinos... $\endgroup$ – CuriousOne Aug 14 '14 at 20:42

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