Caption: Galaxy rotation curve for M33 (AKA the Triangulum Galaxy) superimposed on a to-scale image of M33---with a discussion of MOND below.

Features:

1. M33 with the observed rotation curve and the one expected from observed baryonic matter.

2. The discrepancy between the two rotation curves is take as evidence for the theory of dark matter which consists of a gas of weakly interacting exotic hypothetical particles or primordial black holes (PBHs) in the asteroid window for primordial black hole masses (∼ 10**17--10**23 g) (see Black hole file: black_hole_primordial.html).

   The dark matter is estimated to be ∼ 22 % of the observable universe mass-energy and ∼ 5.5 times more abundant than baryonic matter and ∼ 55 time smore abundant than baryonic matter baryonic dim matter.

3. For a full explanation of this figure, see file galaxy_rotation_curve_m33.html.

4. A counter theory to dark matter is MOND which we discuss below.

MOND (MOdified Newtonian Dynamics):

1. It is possible that there is NO dark matter (i.e., exotic particle dark matter)---or less than the theory of dark matter usually requires---and that both general relativity and Newtonian gravity need correction for very small accelerations: i.e., below ∼ 10**(-10) m/s**2.

   Such low accelerations are unmeasurable in the laboratory, but occur in the outer parts of galaxies and in galaxy clusters, where discrepancies in motions have conventionally been ascribed to dark matter.

2. The theory that dispenses with dark matter---or at least some of it---is MOND.

   Of course, MOND is also a counter theory to our conventional gravity theories: general relativity and Newtonian gravity.

3. We will NOT deeply into the arcane subject of MOND---but we'll go shallowly into it.

   MOND has been around since 1983.

   It's had its had its ups and downs:

   Ups:

   1. Simple MOND theories work well for galaxies and obviate the need for dark matter (e.g., Chan 2013, arXiv:1310.6801, p. 2).

   2. A relativistic MOND theory has been developed (Bekenstein, J. D. 2004, An Alternative to the Dark Matter Paradigm: Relativistic MOND Gravitation, astro-ph/0412652) that makes MOND consistent with the relativity postulate.

   3. There have been many searches for the dark matter particle and there is NO definite detection. Mostly there have been completely null results (e.g., Moskowitz, 2013, Scientific American.)

      The MOND advocates can say show me the dark matter particle if it is real. If you can't, we've got MOND.

   Downs:

   1. MOND theories do NOT work well for galaxy clusters, and so they still MAY need dark matter (e.g., Chan 2013, arXiv:1310.6801, p. 2).

   2. The explanation of Bullet Cluster (which consists of colliding galaxy clusters) and other interacting galaxy systems (Harvey, D. et al. 2015, Science, 347, 1462) highly favors dark matter over MOND.

   3. All the good results MOND gives may be obtainable just by assuming a plausible distribution of dark matter (Chan 2013, arXiv:1310.6801).

   4. There have been some dashed hopes for finding the dark matter particle:
      1. Circa 2015, research groups at Large Hadron Collider (LHC) reported a possible ∼ 750 GeV particle (i.e., ∼ 800 proton-mass particle) which if it exists---a very big if---could be or could be related to the dark matter particle.

         However, further data from 2016 suggested that the 2015 results were a just a statistical fluctuation (see Wikipedia: 750 GeV diphoton excess).

      2. The DAMA/NaI experiment and its successor the DAMA/LIBRA experiment have reported possible exotic hypothetical particle since 1997 that could be the dark matter particle (see Wikipedia: DAMA/NaI/ Results).

         However, since circa 2022 the DAMA/LIBRA experiment results have NOT been confirmed and may be close to being discounted definitively (see G. Adhikari, et al., 2022, arXiv:2104.03537, COSINE-100 article).

4. If MOND turns out to be true, it would cause a revolution in our ideas about about gravity, the large-scale structure of the universe, and cosmology.

   But the odds seem against MOND.

   In fact, it seems that most experts in the field of galaxies, large-scale structure of the universe, cosmology believe MOND is probably a wrong theory.

   But they would also probably say that it is a good counter theory to the more accepted dark matter theory---it keeps demanding that dark matter explain things better---it's a sort of devil's advocate theory---for Himself, see the figure below (local link / general link: dore_satan.html).

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5. In any case, the MOND people keep wiggling---or as Charles Darwin (1809--1882) would say "wriggling" (see Eiseley 1961, p. 115, 295; Loren Eiseley (1907--1977)---Darwin was fond of worms: see Wikipedia: Darwin from Insectivorous Plants to Worms.

   In fact, MOND is the Dracula of theories---it always rises from the dead. See the abode of Dracula in the figure below (local link / general link: dracula_cover.html).

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6. Of course, it could be that we live in the worst of all possible worlds in which there is MOND, dark matter particles, and primordial black holes (PBHs) as another form of dark mattter.

   Occam's razor disfavors more than one theory being right---but Occam's razor is only a philosophy of science rule, NOT a law of nature.

7. Also, of course, all theories could be wrong---but then What is truth?.