So a question for the knowledgeable on this subject. Over the last few years, we continually keep seeing discoveries large number of intra and extra solar system astronomical bodies that do not radiate at high luminosities or frequencies. Presumably all these add to the mass of galaxies.
How much of the dark matter hypothesis is dependent on observations of rotation of high luminosity bodies? If we assume there are large numbers of these non luminous bodies distributed between stars, does the necessity for postulating exotic dark matter go away?
No, it doesn't help. Brown dwarfs as dark matter was a somewhat mainstream theory in the late 90s under the name MACHOs ("MAssive Compact Halo Objects"), but a seminal 2006 study of colliding galaxy dynamics in the "Bullet Cluster" definitively ruled out that possibility:
The distribution of mass in this cluster shows that dark matter does not obey the same physical laws as baryonic matter. Baryonic matter (including interstellar dust, planets, brown dwarfs and stars) self-interacts through electrostatic friction, transforming kinetic energy into heat. Dark matter doesn't interact with baryonic matter in this way, and passes right through without being slowed down by friction. So whatever it's made out of, it can't be protons and electrons.
No, this was not the conclusion of the Bullet Cluster observation.
In the Bullet Cluster, the dark matter and the clumped baryonic matter (like stars, planets, and putative MACHOs) both pass through the collision, unaffected. (For the dark matter, it's because it's non-interacting. For the clumped baryonic matter, it's because there are no long-range forces and it's vanishingly rare for two stars or planets to hit each other.) It is only the plasma (which is baryonic but not clumped) that is slowed down by the collision, due to the long-range electromagnetic forces between charged particles. Since the plasma constituted the bulk of the baryonic matter in the collision by mass, outweighing the stars, this observation is thought to rule out theories that explain missing mass by modifying gravity at long distances (though this interpretation is disputed). The Bullet Cluster is not evidence against MACHOs because they would have followed the exact same trajectory as non-baryonic dark matter.
From the abstract:
> Due to the collision of two clusters, the dissipationless stellar component and the fluid-like X-ray emitting plasma are spatially segregated....we create gravitational lensing maps which show that the gravitational potential does not trace the plasma distribution, the dominant baryonic mass
component, but rather approximately traces the distribution of galaxies. [A] spatial offset of the center of the total mass from the center of the baryonic mass peaks cannot be explained with an alteration of the gravitational force law, and thus proves that the majority of the matter in
the system is unseen.
MACHOs are generally taken to be ruled out by gravitational microlensing results, not the gravitational (macro)lensing seen in the Bullet Cluster.
Dark matter is going away, regardless, because it is not necessary to explain galaxy rotation curves.
https://www.youtube.com/watch?v=PL0ewiwqoTw
Dark matter was always just a way to fudge what was observed. It is hard to believe most physicists, astronomers and cosmologists still believe in and are still looking for something that never existed.
My understanding is that MOND is essentially dead at this point and that there aren't any other promising alternative theories left on the table.
I enjoyed a podcast on the topic by the highly respected physicist Sean Carroll (along with his guest, MIT physicist Lina Necib). It was very interesting and insightful. They cover in-depth why we are so confident that dark matter is a thing that exists.
Simulation in video I linked is not MOND. Dark Matter is postulated due to observations of galaxies in isolation. BUT galaxies are not gravitationally isolated. They gravitationally affected by all the other galaxies in their neighborhood. The video and simulation is pretty straight forward. Dark Matter is simply unnecessary to explain galaxy rotation curves.
Even if you are correct about the galaxy rotation curves -- which everything I've read and heard leads to me believe is rather unlikely -- those curves are only one of several pieces of evicence that point to the existence of dark matter. The wikipedia page list many of these: https://en.wikipedia.org/wiki/Dark_matter#Observational_evid...
I only mentioned MOND as it is one of the more widely-known plausible alternatives that have been pursued.
And I and the video creators are only claiming dark matter is not needed to explain galaxy rotation curves. Whatever other spaghetti lumped into the need for dark matter, have at it... but galaxy rotation curves are explained quite neatly looking at two things: 1) dark matter was initially postulated and never reexamined because galaxies were observed as gravitationally isolated and 2) when not unnaturally gravitationally isolating galaxies from their neighborhood group, the rotation curves make precisely exact and perfect sense
How much of the dark matter hypothesis is dependent on observations of rotation of high luminosity bodies? If we assume there are large numbers of these non luminous bodies distributed between stars, does the necessity for postulating exotic dark matter go away?