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MACHO's and RAMBO's are both baryonic (and leptonic) forms of matter that can't be observed by their nature. They barely emit or reflect light. Black holes, neutron stars, or brown dwarfs (or groups of them) are examples.

We speak of cold dark matter because the particles constituting them have a relative velocity. They interact with other massive objects almost exclusively by gravity, a most wanted condition for dark matter.

Dark matter is tied to the condition that there has to be five times as much of it than the light, normal matter.

Why shouldn't dark matter be just dark normal matter? Would they form two blobs of matter after a collision of two heaps of stars, like is seen in the Bullet Cluster? Due to their smallness they could avoid collisions. The white stars in the heaps collided thereby forming a central light blob in the center on the sides of which the two dark matter blobs are situated. The dark objects have shot through to both sides in the direction of the velocities of the two colliding heaps.

The HALO's are hard to detect. But they are there. Who says there aren't enough of them to account for dark matter?

The question is partly resolved in the supposed duplicate. In that question, only brown dwarfs are mentioned. I can't see neutron stars or black holes or maybe if the massive black hole in the center of the galaxies is bigger than supposed (though this wouldn't explain the seeming existence of black matter heaps without light matter).

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  • $\begingroup$ It doesnt take neutron stars and BH's into consideratio. Maybe even the mass of the SMBH in the center is much highr. It cant be observed directly, so who knowa? I just cant imagine other particles than the standard particles to exist. Maybe the currents in the moving stars can even interact with the magnetic field of the milky Way. The electric currents in the stars have a net direction due to their motion around the center which can cause a magnetic pull. $\endgroup$
    – Zweinstein
    Jul 3 at 16:35
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    $\begingroup$ @Zweinstein Neutron stars and black holes were ruled out almost immediately after the galaxy rotation curve problem was discovered. Neutron stars result from the death of moderately large stars, and black holes from the death of very large stars. There aren't anywhere near enough moderately to very large stars to explain away the problem. As the linked question shows, low mass stars / brown dwarfs also don't work as an explanation. One possible ordinary matter explanation is primordial black holes, but that too is dubious. Science does not yet know what dark matter is. $\endgroup$ Jul 3 at 16:42
  • $\begingroup$ @DavidHammen Why are primordial holes dubious? $\endgroup$
    – Zweinstein
    Jul 3 at 16:44
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    $\begingroup$ @Zweinstein They've always been dubious because their is no evidence they exist. Recent discoveries make them even more dubious. For example annualreviews.org/doi/pdf/10.1146/annurev-nucl-050520-125911 . $\endgroup$ Jul 3 at 16:58
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    $\begingroup$ No, the gravitating mass density far exceeds the baryon density and there are several measurements for this. In addition, dark matter is needed to provide seeds for the structure we see today. Lastly, microlensing observations have pretty much ruled out MACHOS. One last point, we can pretty much detect all baryonic matter since it is above absolute zero and have a good census of where all the baryons are. $\endgroup$
    – Astroturf
    Jul 3 at 17:41
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Firstly, there is a requirement for the dark matter to be non-baryonic. There are manifold reasons for this. The most important are the comparison between the universal energy density implied by measurements of the cosmic microwave background (CMB) compared with the baryonic matter density in the universe implied by measurements of the primordial abundances of helium and deuterium. The former suggests that $\Omega_M \sim 0.3$, whilst that latter suggests that $\Omega_b \sim 0.05$. i.e. There seems to be six times as much gravitating matter in the universe than is required to produce the abundances of chemical elements synthesised during the big bang.

Secondly, non-baryonic matter that does not interact with itself or with "normal" matter is required in order to form the galaxies and clusters of galaxies that we see in the universe today. Without that dark matter it is difficult to understand how the very tiny fluctuations that are seen in the cosmic microwave background can grow into the structures we see in the universe today. If this extra matter were baryonic it would have been evident in the CMB.

Now both these bits of evidence apply to the universe as a whole and it is in any case true that the "luminous matter" that we can see and count-up may still fall short of the amount of baryonic matter that is supposed to be present (though only by a factor of 2 or so). For that reason it is still possible to hypothesise that the dark, gravitating matter in our Galaxy, that is required to explain the flat rotation curve, could be in the form of something baryonic that does not emit much light. The candidates would be very low-mass stars and brown dwarfs, old white dwarfs, neutron stars, black holes, bricks, lost golf balls etc.

These possibilities (well maybe not the lost golf balls) were taken very seriously in the 1980s and 1990s. One of the problems with these hypotheses is that the gravitating dark matter needs to be roughly spherically distributed and have a radial distribution that is much larger than the distribution of the luminous matter. This immediately gives an "origins problem". Why would these dark objects be distributed very differently to the normal matter? After all, we think low mass stars and brown dwarfs are produced like other stars and the white dwarfs, black holes and neutron stars are the endpoints in the lives of stars more massive than the Sun that are distributed mostly in a disc.

Nevertheless, people looked for them. The main tool was the microlensing surveys. By staring at fields towards both the Galactic bulge and Magellanic clouds, the idea was to constrain what populations of "massive compact halo objects" (MACHOS) there could be, by looking for the magnification events that occur when such objects pass directly in front of a background star. These experiments (and there were several) basically ruled out MACHOS as providing anything but a small proportion of the Milky Way dark matter (e.g. Tisserand et al.'s (2008) headline result was that MACHOS in the range $6\times 10^{-8} < M/M_\odot <15$ were ruled out).

Thus a population of white dwarfs, neutron stars, black hole, brown dwarfs or other stars in this mass range are ruled out as contributing significantly to dark matter in the Milky Way and the the other evidences that non-baryonic dark matter is required rule them out as contributing more than a minority of dark matter from that point of view too.

Primordial black holes are a different thing altogether. These would be classed as "non-baryonic" dark matter, since they formed before the epoch of nucleosynthesis. They could in principle lie outside the mass ranges probed by the microlensing surveys and so long as they were not too small, would not have evaporated. The status of this idea is unclear. You can follow the summary and references at https://en.wikipedia.org/wiki/Primordial_black_hole and look at the review by Carr & Kuhnel (2020). However, the Bullet Cluster observations actually have little to say about the nature of the dark matter, only that it is difficult to describe such observations with modified gravity theories.

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The estimates for galaxy masses rely heavily on the mass-luminosity relationship, which is however quite uncertain for low mass stars. From the data in http://adsabs.harvard.edu/full/2004ASPC..318..159H one finds that masses for low mass stars deviate from the theoretical mass-luminosity curve by a factor 2 or more. Considering that about 50% of the stellar mass in a galaxy consists of low mass stars (red dwarfs), at least part of the 'dark matter' could thus be explained by incorrect mass estimates for those stars.

//Edit in view of the comment below://

First of all, there are numerous values for the dark/visible matter ratio floating around. 10x is really only the upper limit. Secondly, the paper I quoted made a rather conservative estimate with 10% error for the galactic stellar mass. From their data it is quite apparent that many low mass stars are about 1-2 absolute visual magnitudes below the theoretical curve, which translates into a 3-4 times higher mass (even considering the shallower mass-luminosity power law for small masses). Even if the average for all low mass stars is only 2x higher, this is a 100% error and therefore a 50% error for the whole mass of the galaxy.

On top of this there may be other errors. For instance, galactic rotation curves are usually obtained by measuring gas velocities not stellar velocities, and gas velocities can be affected by electromagnetic fields during stages of ionization (which would also be consistent with the high energy gas outflow from galaxies).

Furthermore, galaxies have been observed that show little to no evidence of dark matter at all (see https://en.wikipedia.org/wiki/NGC_1052-DF2 ).

So I don't think that the accuracy of observations as well as theoretical models is anywhere good enough yet to arrive at conclusive results here.

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  • $\begingroup$ There is a factor of >10 more gravitating, "dark matter" than "luminous matter". How can an unspecified uncertainty in 50% of the mass of the luminous matter lead to anything like a factor >10 increase in the mass of that luminous matter? The reference you cite says the error in the total stellar mass could be 10%. i.e. 10% of something that is too small by a factor $>10$. $\endgroup$
    – ProfRob
    Jul 4 at 14:42
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    $\begingroup$ @ProfRob Please see my edited answer $\endgroup$
    – Thomas
    Jul 4 at 17:12

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