1/ I was restricting my comments to dark matter, sorry if you read a comment about dark energy into my reply; probably I should have made it explicit that I was

1/ I was restricting my comments to dark matter, sorry if you read a comment about dark energy into my reply; probably I should have made it explicit that I wasn't touching your "fudge factor" comment. Coincidentally, Sabine Hossenfelder has just published this in the past day or so: http://backreaction.blogspot.com/2019/11/what-is-dark-energy.html http://backreaction.blogspot.com/2019/11/what-is-dark-energy... and you might want to take your comment about "fudge factor" there, although I'd bet a doughnut without looking that at least two other assiduous commentators there have already done so. Hopefully someone in there will discuss Jeans instability and the Raychadhuri focusing theorem if it gets sufficiently technical: the latter was a later discovery which (if he had been aware of it) would have led Einstein (and practically everyone else) more quickly away from a steady-state cosmology and towards something like Einstein-deSitter or Einstein-Friedmann as a way of solving the problems arising in the former. One should also bear in mind that until 1922 M31 (then the "Great Andromeda Nebula") was not known to be comparable in size to the Milky Way nor at distances of more than a few thousand light years (as opposed to the modern figure of ~2.5 million), and that it wasn't until several years later that it became known that there are lots of galaxies in the sky -- this was all after the cosmological constant was introduced by Einstein in 1917. 2/ I will restrict myself to just: "I'm not sure if I agree with the assertion that TeVeS is an extension of GR." Bekenstein's tensor-vector-scalar gravitation (TeVeS) is a theory of gravitation, like MOND and GR. You're right that as originally formulated it's not laying a field on top of GR, but like numerous theories of gravitation it was found to be inconsistent with observation. In particular TeVeS does not allow for long-lived stars, and those appear in our sky in abundance. We also have lots of Einstein lensing data that conflicts strongly with TeVeS predictions. Amusingly one fix proposed for TeVeS for the "cosmic shear" weak gravitational lensing is to add a hidden mass term in galaxy clusters, with a specific proposal for a WIMP. (This doesn't fix TeVeS's other difficulties). This is dealt with in Famaey & McGaugh §7.4 (wherein there is a delightful summary of a generalization of TeVeS: "... a tensor-vector-scalar theory with an Einstein-like metric, an Einstein-Aether-like unit-norm vector field, and a k-essence-like scalar field", which is close to saying it's "just GR with two extra fields that you are free to place on either side of the Einstein Field Equations" as you'll likely read from such non-particle-DM academics), and on textbook treatments of the Paramaterized Post-Newtonian (PPN) formalism (a convenient table is here https://en.wikipedia.org/wiki/Alternatives_to_general_relativity#PPN_parameters_for_a_range_of_theories https://en.wikipedia.org/wiki/Alternatives_to_general_relati... and surrounding text). Many of the PPN parameters are chosen so that they can be individually tested; hackernews user ISL does that for a living! 2(b)/ restricting to "the black holes in the center of a galaxy are more massive, higher density of interstellar medium and gas" The work of http://www.astro.ucla.edu/~ghezgroup/gc/ http://www.astro.ucla.edu/~ghezgroup/gc/ and others put strong limits on the stress-energy in the central parsec, and more broadly in the core: your approach does not work in the Milky Way at all, and yet observations are increasingly consistent with a dark matter halo (see https://en.wikipedia.org/wiki/Dark_matter_halo#cite_note-32 https://en.wikipedia.org/wiki/Dark_matter_halo#cite_note-32 and cite note 33 ibid., which review the Milky Way's rotation curve) 2(c)/ "you've mentioned MOND is a flawed theory". The theory itself is fine, it's just not a good match for extragalactic observations. I have zero problem in using pure Milgromian MOND in studies of the doppler shifts of molecular gas clouds in LSB elliptical galaxis, for instance. But it doesn't work at all in cosmology, and is grossly wrong in the solar system and in mergers of massive compact objects, and "fixes" for those regimes are even more unwieldy than the GR-based linearizations and other GR-based post-Newtonian expansion techniques already in use. Finally, "wind back the arrow of time" What does that mean? From context I think you are just saying that large scale structure formation is not adequately explained by theories that lack some form of dark matter, but "the arrow of time" means something to cosmologists (and physicists generally).