Fermi and dark matter

[Michael Mozina]

Are you kidding ?

I have been linking to the Wikipedia article that states that the ICM is a bunch of stuff - mostly ions.

And?

FYI, you've still never explained how you intend to differentiate between "non baryonic dark matter" and a hand sized lump of coal! That's the whole flaw in your argument. Any and all types of "clumpy material", including "clumpy dense clouds of heavier materials" would pass right on through your "collision process".

 

[Reality Check]

And?

FYI, you've still never explained how you intend to differentiate between "non baryonic dark matter" and a hand sized lump of coal! That's the whole flaw in your argument. Any and all types of "clumpy material", including "clumpy dense clouds of heavier materials" would pass right on through your "collision process".

.
I do not have to differentiate between non-baryonic dark matter and MACHOs ("lumpy stuff") because astronomers have techniques to detect "lumpy stuff". They have not detected "lumpy stuff" around the Milky Way. Thus dark matter is not "lumpy stuff".

If you have some other candidate for "lumpy stuff" then what is it?



If you just mean chunks of matter floating out in between galaxies then

• Where did they come form?
• Why are they not visible?

ETA


You may want to read Starts With a Bang's clear description of Dark Matter in his blog entries (especially Part II).

• Dark Matter Part I: How Much Matter is There?
• Dark Matter Part II: How much Normal Matter is there?
• Dark Matter Part III: Dark Matter or Modified Gravity?
• Dark Matter Part 3.5: When Clusters Collide!
• Dear MOND: Time for a New Song! (Dark Matter pt. IV)

ETA2
Think about what you are implying with your "lumpy stuff":

• The total mass of a galactic cluster can be measured using gravitational lensing.
• Various techniques are used to estimate the mass of the galaxies in the cluster. They come out to ~2% or the total mass.
• The X-ray spectrum of the iSM allows an estimate of its mass. This comes to ~14% of the total mass.
• Thus your "lumpy stuff" must consist of 84% of the mass of the cluster or 42 times the mass of the galaxies in the cluster.

 

[Reality Check]

Certainly.

http://www.nature.com/nature/journal/v451/n7175/abs/nature06490.html

So we have a nice paper titled "An asymmetric distribution of positrons in the Galactic disk revealed by gamma-rays".


Unfortunately citing it just reveals your ignorance of the Fermi results. The Fermi results are not about an asymmetric distribution of positrons in the Galactic disk. They are about:

• A gamma-ray haze and
• A feature of the gamma-ray spectrum.

 

[Michael Mozina]

.
I do not have to differentiate between non-baryonic dark matter and MACHOs ("lumpy stuff") because astronomers have techniques to detect "lumpy stuff". They have not detected "lumpy stuff" around the Milky Way. Thus dark matter is not "lumpy stuff".

http://arxiv.org/abs/astro-ph/0001272
You seem to be in the dark about dark matter, particularly the lumpy kind.

 

[Michael Mozina]

Unfortunately citing it just reveals your ignorance of the Fermi results. The Fermi results are not about an asymmetric distribution of positrons in the Galactic disk. They are about:

• A gamma-ray haze and
• A feature of the gamma-ray spectrum.

I take it you didn't read DazzaD's link?

http://www.symmetrymagazine.org/breaking/2009/11/05/dark-matter-at-the-fermi-symposium/

At the end of his talk, when the moderator asked if there were any questions, an audience member immediately went to the microphone and said the excess could be explained another way – namely inverse Compton scattering (which is when energized electrons strike photons and boost their energies) created by a cosmic bubble very near our solar system. Weiner responded quite pleasantly, “Yes, and we’re looking forward to discussing that.”

The person who spoke up at the end of Weiner’s talk was referring to work by Jean Marc Casandjian and Isabel Grenier of CEA Saclay, which was presented in a poster. Casandjian and Grenier, representing the Fermi collaboration, say the excess gamma rays fit the already known shape of the LOOP 1 cosmic bubble. LOOP 1 might be the remnants of an exploded supernova or other massive event that left a radiating bubble about 100 kiloparsecs from the sun (we are surrounded by a similar bubble called the local bubble). LOOP 1 lies directly in our line of sight to the galactic center, and radiation from the bubble interferes with our view of the busy galactic plane. Casandjian hopes the analysis he is doing will allow researchers to subtract the effects of LOOP 1 and get a much clearer view of the Milky Way and the galactic center. There is more work to be done on their analysis before it is ready for publication.

In response to the notion that the gamma-ray excess could be something other than LOOP 1, Casandjian says other analyses did not take the time to extract the detailed structure of the gamma-ray emissions, but instead saw them as a general haze. From the detailed map that he and Grenier constructed, Casandjian said he believes the emissions are a very good fit to the known structure of LOOP 1, which can also be seen in radio frequencies. Still, he acknowledged that he could not claim to understand everything going on with the gamma ray excess.

Unlike the observations of hard gamma rays from the core, I'm not personally aware of anyone who has tried to attribute that specific gamma ray haze or "feature" to "dark matter", but perhaps you know something I do not? You evidently cannot be referring to this paper because those authors took great precaution to *NOT* claim it was related to 'dark matter'.

http://arxiv.org/abs/0910.4583

There have been speculations that the microwave haze could indicate new physics, such as the decay or annihilation of dark matter, or new astrophysics. We do not speculate in this paper on the origin of the haze electrons, other than to make the general observation that the roughly spherical morphology of the haze makes it difficult to explain with any population of disk objects, such as pulsars.

FYI, Thompson and Compton scattering effects are likely to scatter light in lots of directions and produce a general "haze" sooner or later. A "haze" isn't surprising, nor would it be surprising if that "circular feature" you're evidently talking about is related to the LOOP1 cosmic bubble.

 

[DazzaD]

When they say:

Casandjian says other analyses did not take the time to extract the detailed structure of the gamma-ray emissions, but instead saw them as a general haze

They are not really implying that the other studies were rubbish and were unable to fit the data to the model that has dark matter, they just mean that they were unable to collect and analyse the data, that time round, in such a way to even TEST if it fits their data model.

The scientist in question here is just doing what a good scientist does, he is proposing that maybe the observations COULD be explained, in this one particular instance, by the Loop1 Bubble... but he HASNT done that in detail yet, it is a suggestion.

I would be wary of reading too much into a suggestion at a symposium, if you were too get too carried away doing that then we would have about 50 new theories at every single symposium.

For the time being it is just scientists batting ideas back and forth, not an outright denial of the standard model which includes such a large portion of dark matter, far from it.

I just wanted my position to be made clear lest anyone assume that in this particular discussion I am completely on the side of the "plasma cosmology" (I am not sure what MM would prefer I call his preferred theory).


While we are on the topic, I do find it amusing when some people suggest that astrophysicists don't study plasmas when they are the bread and butter of much of their work

 

[Tubbythin]

Certainly.

http://www.nature.com/nature/journal/v451/n7175/abs/nature06490.html

Tell me how your "dark matter" solution is "simpler".

Now we might be getting somewhere. I have no particular problem with the idea that the excess gamma rays aren't from dark matter. I'm not an expert and certainly not the best to evaluate and compare the hypothesise. I have a problem with the misuse of Occam's razor to discriminate between two theories when either (or both) arguments do not describe the facts. Arguments beginning "There have to be..." will never ever ever explain the facts.

Sorry but a numerical "theory" related to "magic stuff" isn't much of an "explanation" IMO to begin with, so how in the heck are you going to tell me that DM solutions are better than a binary star solution to the exact same observation?

I'm not. I don't know. But suggesting that dark matter doesn't exist because it might not be the cause of one observational phenomenon is blindly forgetting about all the other observational phenomena that cannot be explained by LMXBs.

 

[Reality Check]

I take it you didn't read DazzaD's link?
http://www.symmetrymagazine.org/breaking/2009/11/05/dark-matter-at-the-fermi-symposium/

No I hadn't - I have been too busy with your "lumpy stuff" stuff.

Unlike the observations of hard gamma rays from the core, I'm not personally aware of anyone who has tried to attribute that specific gamma ray haze or "feature" to "dark matter", but perhaps you know something I do not?

Many people have "attributed" that specific gamma ray haze or feature to dark matter, including the authors of the paper (as you quoted). This attribution has included other possibilities.
For example: Has Fermi Seen New Evidence for Dark Matter?

No one that I am aware of has attributed that gamma haze solely to dark matter, i.e. ruling out other possibilities.

You evidently cannot be referring to this paper because those authors took great precaution to *NOT* claim it was related to 'dark matter'.

http://arxiv.org/abs/0910.4583

As above - the authors pointedly do not claim that what we see must arise from any cause (including dark matter). That is what you highlighted.

FYI, Thompson and Compton scattering effects are likely to scatter light in lots of directions and produce a general "haze" sooner or later. A "haze" isn't surprising, nor would it be surprising if that "circular feature" you're evidently talking about is related to the LOOP1 cosmic bubble.

As DazzaD says - the comment is from a symposium. The work from Jean Marc Casandjian and Isabel Grenier was presented in a poster session, i.e. typically a description of unpublished work that is in progress (at least that is what I presented im my poster sessions). That Marc Casandjian and Isabel Grenier's work is not ready for publication is even in what you quote.

 

[Reality Check]

http://arxiv.org/abs/astro-ph/0001272
You seem to be in the dark about dark matter, particularly the lumpy kind.

You seem to be in the dark about massive astrophysical compact halo objects, particularly that there is not enough of them to account for the dark matter in galaxies and they really badly fail to account for the dark matter between galaxies.

I have read the The MACHO Project: Microlensing Results from 5.7 Years of LMC Observations paper that was published in 2000.
It reported enough MACHOs to account for ~20% of the dark matter in the Milky Way.
EROS2 collaboration published this paper in 2007 and found ony ~8% of the dark matter accounted for in the Milky Way.
Limits on the Macho Content of the Galactic Halo from the EROS-2 Survey of the Magellanic Clouds.

Of course you must know by now that most dark matter is measured to be outside of galaxies (remember gravitational lensing?). And only an truly ignorant person would not know that the halo part of MACHO refers to galactic halos which are part of galaxies.

Massive compact halo object

A MACHO may be detected when it passes in front of or nearly in front of a star and the MACHO's gravity bends the light, causing the star to appear brighter in an example of gravitational lensing known as gravitational microlensing. Several groups have searched for MACHOs by searching for the microlensing amplification of light. These groups have ruled out dark matter being explained by MACHOs with mass in the range 0.00000001 solar masses to 100 solar masses. One group, the MACHO collaboration, claims to have found enough microlensing to predict the existence of many MACHOs with mass of about 0.5 solar masses, enough to make up perhaps 20% of the dark matter in the galaxy.[1] This suggests that MACHOs could be white dwarfs or red dwarfs which have similar masses. However, red and white dwarfs are not completely dark; they do emit some light, and so can be searched for with the Hubble Telescope and with proper motion surveys. These searches have ruled out the possibility that these objects make up a significant fraction of dark matter in our galaxy. Another group, the EROS2 collaboration does not confirm the signal claims by the MACHO group. They did not find enough microlensing effect with a sensitivity higher by a factor 2.[2] Observations using the Hubble Space Telescope's NICMOS instrument showed that less than one percent of the halo mass is composed of red dwarfs.[3][4] This corresponds to a negligible fraction of the dark matter halo mass. Therefore, the missing mass problem is not solved by MACHOs.

http://xxx.lanl.gov/abs/astro-ph/0607207

 

[Reality Check]

First asked 10 November 2009:
We know that MACHOs are not common enough to explain even the amount of dark matter inside galaxies (Massive compact halo object). By definition they cannot account for dark matter outside of galaxies.

If you have some other candidate for "lumpy stuff" then what is it?

 

[edd]

And?

FYI, you've still never explained how you intend to differentiate between "non baryonic dark matter" and a hand sized lump of coal! That's the whole flaw in your argument. Any and all types of "clumpy material", including "clumpy dense clouds of heavier materials" would pass right on through your "collision process".

If you want interstellar coal you have to identify how those objects form and how they get distributed in the right way and identify why they can't form larger objects such that we would detect and after all that explain how you're going to fix big bang nucleosynthesis and the CMB power spectrum to allow for all these excess baryons.

In other words, we have differentiated between them and we've discussed the measurement of the baryon fraction already, upthread.

 

[Michael Mozina]

If you want interstellar coal you have to identify how those objects form and how they get distributed in the right way and identify why they can't form larger objects such that we would detect and after all that explain how you're going to fix big bang nucleosynthesis and the CMB power spectrum to allow for all these excess baryons.

In other words, we have differentiated between them and we've discussed the measurement of the baryon fraction already, upthread.

There's something a wee bit amusing about you expecting me to explain how cometary and particulate type materials might have accumulated in that area and be distributed in that area, yet none of you folks can explain how 'dark matter' would come to be there and remain 'spread out'. Keep in mind that such objects strike our own atmosphere every single day. I can't tell you with absolute certainty where every "shooting star" comes from, but undoubtedly such material exists in nature.

I personally do not believe that all matter and energy was ever collected to a single "lump". I also happen to believe that various elements mass separate in suns and I believe that hydrogen and helium are most abundant because they most easily escape the gravity wells of suns. I therefore don't really care about "your" nucleosynthesis problem.

 

[Michael Mozina]

Now we might be getting somewhere. I have no particular problem with the idea that the excess gamma rays aren't from dark matter.

What I do not fullly understand is why you believe that *any* gamma rays come from "dark matter". Have you ever seen it demonstrated?

Anything and everything "could" be a "cause", but only certain physical processes are *known* to release gamma rays. Now I can't honestly say the binary stars release more gamma rays, but since all stars emit them, it's certain that binary stars *do* emit them too, perhaps in larger quantities. It's certainly not a 'huge leap of faith' to believe that stars emit gamma rays or even that binary pairs in the core emit more of them. I can see own own sun in the Fermi images.

I'm not an expert and certainly not the best to evaluate and compare the hypothesise. I have a problem with the misuse of Occam's razor to discriminate between two theories when either (or both) arguments do not describe the facts. Arguments beginning "There have to be..." will never ever ever explain the facts.

My point is that there could be any number of legitimate theories to explain gamma rays that do not involve any new type of matter, nor rely upon any unproven assertions. Past papers that have attributed these gamma rays from 'dark matter' in the core essentially had the rug pulled out from under them in this paper. There's no need for an exotic material and therefore any theory that relies upon exotic material is already at a disadvantage in terms of "simplicity", not to mention "empirical physics".

I'm not. I don't know. But suggesting that dark matter doesn't exist because it might not be the cause of one observational phenomenon is blindly forgetting about all the other observational phenomena that cannot be explained by LMXBs.

I'm quite certain that MACHO forms of "dark matter" exist in nature. I am equally sure that cometary and meteorite material exists in nature too. I can see their effect on nature every night when I look at the stars. I'm pretty sure that neutrinos have mass and that mass makes up some of the "missing mass" in the universe too. I do not however have any evidence that exotic forms of matter exist or have any effect on me or nature. I therefore "lack belief" in SUSY particles with ad hoc properties. IMO any theory that requires exotic particles is making an "extraordinary" claim and as such it requires extraordinary support. Missing mass does not necessarily equate to "exotic mass", and giving hypothetical particles hypothetical 'properties' is just silly IMO. Such a theory will necessarily have to be "last on the list" in terms of serious consideration, and *any* theory that doesn't require hypothetical new forms of matter is automatically going to be more 'scientifically credible" in terms of empirical physics.

 

[Michael Mozina]

No I hadn't - I have been too busy with your "lumpy stuff" stuff.

Many people have "attributed" that specific gamma ray haze or feature to dark matter, including the authors of the paper (as you quoted).

I respectfully suggest you reread the quote. The authors specifically chose *not* to speculate on the origin of these event.

There have been speculations that the microwave haze could indicate new physics, such as the decay or annihilation of dark matter, or new astrophysics. We do not speculate in this paper on the origin of the haze electrons, other than to make the general observation that the roughly spherical morphology of the haze makes it difficult to explain with any population of disk objects, such as pulsars.

While the authors note that "some people" have "speculated" on this topic, they specifically chose not to do so. You can't then turn around and ignore the fact that they chose *NOT* to attribute this process to anything in particular and now try to claim "dark matter did it' based on this particular paper. They never made that claim! That is *YOUR* claim not theirs. Your claim lacks both empirical support and it lacks that ever beloved "quantification" thingy you folks so value.

No one that I am aware of has attributed that gamma haze solely to dark matter, i.e. ruling out other possibilities.

I haven't seen you produce a paper that attributed the haze to dark matter yet! To my knowledge the "most likely culprit" of a synchrotron radiation process would be the EM field which just so happens to be involved in gamma ray discharge processes on Earth.

http://en.wikipedia.org/wiki/Synchrotron_radiation

You don't have an empirical leg to stand on, and your authors did not make any claims about the "cause" of these emissions.

As above - the authors pointedly do not claim that what we see must arise from any cause (including dark matter). That is what you highlighted.

That's my whole point. You're sitting here trying to claim that 'dark matter' has something to do with these "features", but no paper you've produced thus far actually makes that claim. The only "feature" I've ever seen attributed to "dark matter annihilation" in a paper can also be explained by simple binary star populations and those so happen to produce the "shape" we're looking for as well. In other words there is a "better" scientific explanation for gamma rays than have been provided by 'dark matter annihilation' proponents.

As DazzaD says - the comment is from a symposium. The work from Jean Marc Casandjian and Isabel Grenier was presented in a poster session, i.e. typically a description of unpublished work that is in progress (at least that is what I presented im my poster sessions). That Marc Casandjian and Isabel Grenier's work is not ready for publication is even in what you quote.

Ok. FYI, you still have yet to produce a single paper attributing the 'features' you're talking about to "dark matter". I have also showed you at least one "better" paper to explain gamma ray signatures in and around the core. Unless you have evidence, or even a mathematical presentation that "dark matter" has any effect on any of these observations, you really haven't got a single leg to stand on.

 

[ben m]

There's something a wee bit amusing about you expecting me to explain how cometary and particulate type materials might have accumulated in that area and be distributed in that area, yet none of you folks can explain how 'dark matter' would come to be there and remain 'spread out'.

Don't get confused: "Michael Mozina doesn't understand" is different than "none of you folks can explain". Accumulating in gravity wells but remaining spread-out within those wells is exactly what you expect for a collection of collisionless particles. This is very well known, MM. When people give talks on the dark matter hypothesis this is, like, the third slide.

I personally do not believe that all matter and energy was ever collected to a single "lump". I also happen to believe that various elements mass separate in suns and I believe that hydrogen and helium are most abundant because they most easily escape the gravity wells of suns. I therefore don't really care about "your" nucleosynthesis problem.

Geez, why are you complaining about dark matter? If you don't agree with any of the conclusions of mainstream astrophysics, then of course you have no reason to believe in dark matter, dark energy, gravitational lensing, the Hubble relation, accretion disks, synchrotron radiation, extrasolar planets, blue straggler stars.

If you're so much of a contrarian that you can't accept stellar interior hydrostatics, which depend on such straightforward physics and math that Jonathan Homer Lane got much of it right in 1870---well, of course you can't accept dark matter. Particle dark matter is an attractive hypothesis because of its agreement with dozens of major aspects of concordance cosmology, none of which you believe or understand.

Particle dark matter is not an attractive hypothesis if astrophysicists have everything else wrong---really, nearly everything---to begin with.

Similarly, BCS theory is not a good explanation for superconductivity if quantum mechanics is wrong. The endosymbiont hypothesis does not explain mitochondria ... if the Bible literalists are right. Plate tectonics is not a good hypothesis geologists are wrong about the Earth being round.

It's like saying, "I am Gene Ray and I believe that the Earth is a cube and time is an cube and spheres are evil. Furthermore, the GPS system doesn't actually use General Relativity."

 

[Michael Mozina]

If you just mean chunks of matter floating out in between galaxies then

• Where did they come form?

• 
  
  I don't really know why you expect me to answer this question, yet give yourself a free pass as it relates to "dark matter", but I'll use the lingo of your previous link:
  
  "The ICM is populated by objects that have gravitational energy released by the formation of the cluster from smaller structures."
  
  

  [*]Why are they not visible?

  
  They are very visible at night. They are called "shooting stars".

 

[Reality Check]

That's my whole point. You're sitting here trying to claim that 'dark matter' has something to do with these "features", but no paper you've produced thus far actually makes that claim.

That's my whole point.
No paper states that the gamma haze must be dark matter annihilation.
No paper states that the gamma haze must *NOT* be dark matter annihilation.
The authors of the paper leave the question of the origin of the gamma haze open. Other scientists have speculated that it could be

• synchrotron radiation or
• dark matter annihilation or
• radiation from LOOP 1 or
• other new physics or astrophysics or
• a combination of the above.

I am not claiming that dark matter has something to do with the gamma haze. I am claiming that dark matter may have something to do with the gamma haze (subject to independent confirmation that it annihilates).

One more time:
No one is claiming that dark matter has something to do with the gamma haze. Some scientists are claiming that dark matter may have something to do with the gamma haze.

 

[Reality Check]

I don't really know why you expect me to answer this question, yet give yourself a free pass as it relates to "dark matter", but I'll use the lingo of your previous link:

"The ICM is populated by objects that have gravitational energy released by the formation of the cluster from smaller structures."

They are very visible at night. They are called "shooting stars".

I want to be sure here:
You are asserting that for each galaxy in a galactic cluster there are ~40 galaxies comprised of asteroids.

What happened to the stars that these asteroids formed around?

 

[Michael Mozina]

Don't get confused: "Michael Mozina doesn't understand" is different than "none of you folks can explain". Accumulating in gravity wells but remaining spread-out within those wells is exactly what you expect for a collection of collisionless particles. This is very well known, MM. When people give talks on the dark matter hypothesis this is, like, the third slide.

Even neutrinos are not "collisionless" and you've never demonstrated such things even exist in nature. I'm just supposed to "assume" this with you, yet there is no discussion on how these particles came to be located where they are. The exact same physical processes could be applied to any and all solid materials of a relatively small size that might be present in that region. They would act in a relatively 'collisionless" manner (unless they actually hit something solid) and they would have exactly the same effect as your "collisionless" matter in terms of "passing through" the collision process.

Geez, why are you complaining about dark matter? If you don't agree with any of the conclusions of mainstream astrophysics,

Strawman alert. I didn't say I didn't agree with "any" of their conclusions.

then of course you have no reason to believe in dark matter,

I know you'd love to oversimplify the problem but unfortunately it's just not that simple. I do in fact believe in MACHO forms of 'dark matter", neutrino forms of "dark matter" and "stuff we cannot account for" in galaxies. It's only that 'collisionless" metaphysical brand of 'dark matter' I don't believe in.

dark energy,

Well, I can accept that the universe is 'accelerating', I just lack belief that 'dark energy' has anything to do with accelerating a plasma universe.

gravitational lensing,

Actually, that particular "technique' impresses me personally a great deal.

the Hubble relation,

Actually I tend to favor an "expanding universe" personally, but I tend to think of 'Hubble's law' as more of "Hubble's rule of thumb".

accretion disks, synchrotron radiation, extrasolar planets, blue straggler stars.

I don't recall ever complaining about those ideas.

If you're so much of a contrarian that you can't accept stellar interior hydrostatics, which depend on such straightforward physics and math that Jonathan Homer Lane got much of it right in 1870---well, of course you can't accept dark matter.

FYI, I can't technically see inside the 'crust' so I don't actually reject stellar interior hydrostatics. I just think Birkeland's model better explains the events we observe on the *EXTERIOR* of the sun. I would never claim that current solar theory is "woo" because it is based on known forms of matter and known physical processes. On the other hand I would say that "dark energy" and that "collisionless" brand of dark matter you're talking about are in fact "woo" because they don't exist in nature and cannot be shown to exist in nature. Fusion happens on Earth in "real tangible products". Gravity pulls stuff together and does so here on Earth. Dark energy never does anything here on Earth and to my knowledge nor does any form of "colllisionless matter". When was the last time LIGO saw any signs of "dark matter" just "passing through" the experiment?

Particle dark matter is an attractive hypothesis because of its agreement with dozens of major aspects of concordance cosmology, none of which you believe or understand.

Well, you are right about that point to a great degree. For instance, I don't personally hold any belief about all matter and energy being concentrated into a single "lump". I therefore don't care much about nucleosythesis type arguments that would limit the amount of material that could be "created" in a 'big bang' event. I tend to favor Alfven's notion of a cyclical "bang" that was not a 'creation event' per se, but rather a condensation/expansion of preexisting matter.

I do tend to "understand" most of the arguments made by the mainstream, I simply don't "agree" with all of them.

Particle dark matter is not an attractive hypothesis if astrophysicists have everything else wrong---really, nearly everything---to begin with.

It is more "attractive" than a MOND sort of solution to me personally due to the lensing data. Again, I am not claiming that astronomers got *everything* wrong. That seems to be your own strawman Ben, it's certainly not my opinion.

 

[ben m]

Even neutrinos are not "collisionless" and you've never demonstrated such things even exist in nature.

Look at the neutrino cross section. Calculate how many times an average neutrino will collide with something in 13.7 billion years of orbiting the galaxy. If the number is much less than 1 we call it collisionless. If the number is close to 1 we call it nearly collisionless. Heck, even globular clusters of stars are nearly collisionless.

I'm just supposed to "assume" this with you, yet there is no discussion on how these particles came to be located where they are. The exact same physical processes could be applied to any and all solid materials of a relatively small size that might be present in that region. They would act in a relatively 'collisionless" manner (unless they actually hit something solid) and they would have exactly the same effect as your "collisionless" matter in terms of "passing through" the collision process.

MACHOs would also be collisionless. Black holes would be collisionless. That is why they were considered dark matter candidates---because dark matter is located in places/configurations that can only occur for something collisionless.

The "discussion of how these particles came to be located where they are" is called "galaxy formation" and has been one of the major topics of astrophysics for most of a century.

I know you'd love to oversimplify the problem but unfortunately it's just not that simple. I do in fact believe in MACHO forms of 'dark matter", neutrino forms of "dark matter" and "stuff we cannot account for" in galaxies. It's only that 'collisionless" metaphysical brand of 'dark matter' I don't believe in.

Funny, MACHOs and massive neutrinos ARE collisionless hypothetical objects that there is no experimental evidence for. (Actually, they are collisionless hypothetical objects that there is abundant evidence AGAINST.)