Because the theories that argue that physical laws behave differently at larger distances would have to work similarly for all galaxies. Physicists please correct me if I am wrong in making this conclusion!
Only if competing arguments that account for the rotational speed discrepancy in galaxies fail to explain "galaxies without dark matter", which... I think they do? (I am not 100% sure on this).
Of course occam's razor is only a heuristic, and not a law, but we would prefer a theory that is simpler, and if we have to add an extra level of indirection explaining why some galaxies lack dark matter, then that is less desirable in a theory. Repeating myself, science doesn't necessarily do what we desire.
>Ironically, the lack of dark matter in these UDGs strengthens the case for dark matter, the researchers say. It proves that dark matter is a substance that is not coupled with normal matter, since they can be found separately.
https://medium.com/starts-with-a-bang/mysterious-galaxy-meas...
“Ironically, the discovery of a galaxy without dark matter — amidst a sea of galaxies that require huge amounts of it — helps prove the validity of the dark matter-rich picture of the Universe. Only if normal matter can be separated out from the dark matter and left to form its own structures would such a galaxy be possible.”
These findings do indicate that the Dark Matter theory is increasingly reliable, in that some galaxies are shown to have different densities (& thereby there's nothing wrong with Kepler's Law, and further proof that my "going with the gut" when compared with empirical science is just silly!)
These findings help reinforce that Kepler's Law are working well and seem to indicate that that dark matter is not necessarily tied with matter, but is hanging around in different densities in different galaxies.
> Ironically, the lack of dark matter in these UDGs strengthens the case for dark matter, the researchers say. It proves that dark matter is a substance that is not coupled with normal matter, since they can be found separately.
To me, it's a bit like how our planet is not uniformly the "same stuff", but we have deposits of Gold and Coal and Granite spread around the place in an uneven manner.
Actually Dark Matter is just a way of saying there's something (not necessarily a "substance") we still don't know: the old metaphor of light for knowledge.
Such as?
Another interesting one is Superfluid Dark Matter (first proposed, to my knowledge, here: https://arxiv.org/abs/1507.01019).
Those are just a few, there are probably more theories that I haven't heard about. Whatever the case, Sabine Hossenfelder recently posted three videos about Dark Matter/MOND/Superfluid Dark Matter that I wholeheartedly recommend:
Dark Matter: https://www.youtube.com/watch?v=FN2d2cmi_Gk
MOND: https://www.youtube.com/watch?v=2VNcDoLNJk8
Superfluid Dark Matter (note that it's the theory she supports): https://www.youtube.com/watch?v=468cyBZ_cq4
> Superfluid Dark Matter
As far as I see, from three theories you listed, two are the dark matter theories. The only "alternative" from your list is MOND and as far as I know it has more problems than it can solve:
https://medium.com/starts-with-a-bang/theres-a-debate-raging...
"The tests on the largest scales give us the best tests for dark matter. And these are the ones that dark matter not only universally passes, but that MOND has failed spectacularly for, on every account, for the past 35 years. Among cosmologists*, there is no debate, because there is no alternative to dark matter that reproduces the observed successes."
https://arxiv.org/abs/1112.1320
"The biggest challenge facing MOND today is the shape of the matter power spectrum. The shape depicted in Fig. 1 is related to the acoustic oscillations observed in the CMB. If the Universe is dominated by dark matter, these matter oscillations, dubbed Baryon Acoustic Oscillations (BAO), are highly suppressed as the baryons fall into the potential wells created by dark matter, leaving only percent level traces of the primordial oscillations. In a no-dark matter model, on the other hand, the oscillations should be just as apparent in matter as they are in the radiation. Indeed, Fig. 1 illustrates that – even if a generalization such as TeVeS fixes the amplitude problem – the shape of the predicted spectrum is in violent disagreement with the observed shape."
To to be able to fit with the CMB measurements, at that scale this theory behaves exactly like a normal dark matter, so there is still 5 times more dark matter than a baryonic matter.
So it's just "more complicated" at the galaxy level than a normal dark matter, to fit with these measurements "better" too: it involves an "additional force" there. That also means, considering everything together, it's nothing that should make happy those that consider the normal dark matter "unintuitive."
The supersolid dark matter displaced by a galaxy pushes back, causing the stars in the outer arms of the galaxy to orbit the galactic center at the rate in which they do.
Displaced supersolid dark matter is curved spacetime.
In the Bullet Cluster collision the dark matter has not separated from the ordinary matter. The collision is analogous to two boats that collide, the boats slow down and their bow waves continue to propagate. The water has not separated from the boats, the bow waves have. In the Bullet Cluster collision the galaxy's associated dark matter displacement waves have separated from the colliding galaxies, causing the light to lense as it passes through the waves.
I'd love for climate science research to be more balanced...
Seriously, the "balance" tends to fall towards the theories that have the strongest indications of being on the right track, and that's what's happened with dark matter.
In the video on the only non-dark-matter theory that you linked, MOND, it is pointed out that MOND is known to be wrong, and is considered to at best be an approximation to an unknown theory of modified gravity. MOND itself also has no real theoretical justification, i.e. no explanation for how or why the gravitational equations should be modified.
I'm not a physicist, but this strengthens my personal hunch that dark matter doesn't exist. My heretic theory is that it is just a product of some sort of physical law acting as a "ceiling" in local gravitational force which gets conserved by spreading over large distances.
https://www.forbes.com/sites/startswithabang/2017/02/28/is-d...
http://www.astronomy.com/news/2018/05/the-case-against-dark-...