Caption: A plot showing how the acceleration of orbiting astronomical objects in the outer parts of galaxies with NO dark matter in dark matter halos differs between the prediction of Newtonian physics and that of Milgrom's law (which is simple version of MOND (MOdified Newtonian Dynamics)).

What is MOND?:

1. It is possible that there is NO dark matter and that general relativity and Newtonian gravity and/or inertial mass (i.e., resistance to acceleration) need correction for very small accelerations (i.e., those ∼< 10**(-10) m/s**2 = 10**(-13) km/s**2).

   Such low accelerations are unmeasurable in the laboratory, but occur in the outer parts of galaxies and in galaxy clusters, where discrepancies from the motions implied by Newtonian physics (especially Newtonian gravitation) using only baryonic matter have usually been ascribed to dark matter in dark matter halos. For more on dark matter, see Galaxies file: galaxy_rotation.html.

2. The theory (more exactly the paradigm or grand theory which includes many specific theories) that dispenses with dark matter is MOND. MOND is an abbreviation for MOdified Newtonian Dynamics, but the full name may NO longer be accurate since MOND has evolved considerably since it was first presented in 1983.

   MOND is a counter theory to our conventional gravity theories (general relativity and Newtonian gravity) and/or inertial mass (i.e., resistance to acceleration).

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

   MOND has had its had its ups and downs since first present in 1983.

   Ups:

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

   2. Relativistic MOND theories have been developed (e.g., J.D. Bekenstein, 2004, "An Alternative to the Dark Matter Paradigm: Relativistic MOND Gravitation"; C. Skordis, T. Zlosnik, 2020, "A new relativistic theory for Modified Newtonian Dynamics") that make MOND consistent to some degree with the theory of relativity.

   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 have always been able to say show me the dark matter particle if it is real. If you CANNOT, we've got MOND.

   4. MOND continues to be able to account wonderfully for galaxy rotation curves and some other galaxy features (see, e.g., F. Lelli, 2024, "The empirical laws of galaxy dynamics: from gas kinematics to weak lensing").

   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. Using dark matter, calculations of structure formation (AKA large-scale structure formation) starting from primordial fluctuations in dark matter implied by the cosmic microwave background (CMB, T = 2.72548(57) K (Fixsen 2009)) reproduce the evolution of the large scale structure of the observable universe as it evolves through cosmic time from the recombination era t = 377,770(3200) Jyr = 1.192*10**13 s (z = 1089.80(21)) to the present pretty well.

      In particular, the calculated dark matter halos of galaxies reproduce the observed galaxy rotation curves pretty well (e.g., Chan 2013, arXiv:1310.6801).

      There are discrepancies from the calculations, but those can be attributed to weaknesses in the numerical methods and physical inputs, and also perhaps due to needing evolving dark energy as opposed to the usually used cosmological constant (AKA Lambda, Λ) which can be regarded as the simplest form of dark energy.

      Nothing yet suggests these discrepancies CANNOT be overcome eventually with improvements.

      Structure formation calculations with MOND have NOT yet reproduced the observations nearly as well.

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

   But the odds seem against MOND.

   In fact, most researchers in the field of galaxies, large-scale structure of the universe, and cosmology believe MOND is probably a wrong theory.

   But they would also probably say that MOND 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 Laird Cregar (1913--1944) in Heanen Can Wait (1943 film).

5. In any case, the MOND advocates keep wiggling free of falsification tests---or as Charles Darwin (1809--1882) would say "wriggling" free (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).

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 recommended philosophy of science procedure for research, NOT a law of nature.

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

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