Fermi and dark matter

[Reality Check]

OMG RC, you sure relied on the personal insult a lot in that post. I'll skip the line by line response and cut to the chase.....

*Some* of that "plasma' might indeed be "ionized" in some way. Most "plasmas" however are quite "dusty", meaning the protons and ions picked up an electrons somewhere and now it's a neutral hydrogen or helium or oxygen atom. Some of the "dust" may in fact be "charged" or ionized in some way, but surely not all of it.

The insults seem to be only way to get your attention. I hope that you now realize that the question is about the intra-cluster medium (not solar systems, stars or ISM as you have been going on about for many posts).

Since you seem unable to read the Wikipedia article on the intracluster medium

In astronomy, the intracluster medium (or ICM) is the superheated gas present at the center of a galaxy cluster. This plasma is heated to temperatures of between roughly 10 and 100 megakelvins and consists mainly of ionised hydrogen and helium, containing most of the baryonic material in the cluster. The ICM strongly emits X-ray radiation.
...
Composition
The ICM is composed primarily of ordinary baryons (mainly ionised hydrogen and helium). This plasma is enriched with heavy elements, such as iron. The amount of heavy elements relative to hydrogen (known as metallicity in astronomy) is roughly a third of the value in the sun. Most of the baryons in the cluster (80-95%) reside in the ICM, rather than in the luminous matter, such as galaxies and stars. However, most of the mass in a galaxy cluster consists of dark matter.
Although the ICM on the whole contains the bulk of a cluster's baryons, it is not very dense, with typical values of 10^-3 particles per cubic centimeter. The mean free path of the particles is roughly 10¹⁶ m, or about one lightyear.
The strong gravitational field of clusters means that they can retain even elements created in high-energy supernovae. Studying the composition of the ICM at varying redshift (which results in looking at different points back in time) can therefore give a record of element production in the universe if they are typical.

Emphasis added.
The metallicityof the ICM is 0.06%. That is not "dusty".

Note that in the following atoms refers to both ionized atoms (the majority) and any neutral atoms.

In this case we have clouds of ICM colliding where the mean free path of the atoms is about a lightyear. This means that even if the ICM was not colliding the atoms will travel about a lightyear before colliding. I would expect that the collision would double the density of the ICM, halve the mean free path and so an atom will collides every 0.5 lightyears.

Thus in order for an atom to "pass through" the 1 megaparsec thickness of the ICM it will collide and interact with other atoms at least 3 million times and possibly 6 million times. Scientists know a lot about what happens when atoms of normal matter collide.

• In general they slow down.
• They also heat up and emit radiation.

That is what astronomers see for some of the ICM

• The ICM slows down and forms a blob at the center of the collision.
• The ICM is already emitting X-rays (it is a plasma). The collision produces more X-rays allowing astronomers to map the position of this part of the ICM and see the shock-waves as the galactic clusters collide.

But most of the ICM

• Does not slow down.
• Has not heated up.

According to you this is normal matter that has magically avoided over 3 million possible collisions and just passed through the rest of the ICM. You might have a point if only a millionth part of the ICM passed through. But that is not the case - most of the ICM managed to pass through the rest of the ICM.
You might have a point if only the metallic part of the ICM passed through (0.06% of the ICM is not H or He). But that is not the case - most of the ICM managed to pass through the rest of the ICM.

Other people will conclude that the reason that most of the ICM passed through the rest of the ICM is because most of the ICM does not interact electromagnetically with the rest of the ICM. This means that this part of the ICM is made up of something we have not seen before - weakly interacting massive particles (WIMPs).

 

[Reality Check]

Unless it does, the cited papers you handed me are utterly meaningless and a great waste of time.

That is right.
If dark matter does not emit gamma rays then the cited papers I handed you are utterly meaningless and a great waste of time. That is how science works. If you want to do science then you have to take the risk that you are wrong and are wasting your time (been there done that!).

But ... what if dark matter does emit gamma rays? What if one of the dark matter detection experiments in the labs confirms the DAMA results? What if they go further and measure dark matter interacting with normal matter directly?
The question in a few decades could be which of the authors of these papers gets a Nobel Prize!

You are taking skepisim too far (maybe to bolster your presonal opinion about dark matter).
It is not meant to be a tool to exclude theories beacuse they currently have little evidence.
It is a tool to exclude theories that have no evidence at all and no prospect of any evidence in the future.
Science is a tool to exclude theories that have evidence against them.

 

[Michael Mozina]

The insults seem to be only way to get your attention.

I'd condemn you for it, but oh the hypocrisy if I did.

I hope that you now realize that the question is about the intra-cluster medium (not solar systems, stars or ISM as you have been going on about for many posts).

I'm simply noting that lots of "normal matter" is going to "pass through" the other galaxy. Fine, we'll fixate on the "ICM" specifically but it's not going to help your case one iota.

Since you seem unable to read the Wikipedia article on the intracluster medium

Emphasis added.
The metallicityof the ICM is 0.06%. That is not "dusty".

Before I forget, what exactly do you figure is going to happen to that iron core around the central mass of the galaxy?

FYI, the term "dusty" in reference to a plasma is a reference to non ionized particles in the plasma, not a particular element (like a metal). Not all the atoms will be ionized. Heavier atoms (like that iron around the core) isn't likely to be "fully" ionized, even if it is "partially' ionized, and heavier elements will contain much more mass than a single proton.

What is the percentage of ionized atoms to non ionized atoms? How will ionization affect the "collision frequency"? How will it effect the trajectory of interaction?

Note that in the following atoms refers to both ionized atoms (the majority) and any neutral atoms.

You're treating them exactly the same? Really? By the way, are these ions still connected to the stellar infrastructure that presumably provides the heat for ionization? Surely "neutral" atoms will not act the same as a charged ions, will they?

I think I'll go get a cup of coffee....BRB.

 

[Reality Check]

I'm simply noting that lots of "normal matter" is going to "pass through" the other galaxy. Fine, we'll fixate on the "ICM" specifically but it's not going to help your case one iota.

That is not quite right but you are getting there. The galaxies in the colliding galactic clusters rarely collide so they do "pass through" the cluster.

However

• They are a small part of the mass of the clusters. The ICM has on average 9 times the mass of the galaxies. And dark matter outweighs them both.
• The observations are of the ICM.

Before I forget, what exactly do you figure is going to happen to that iron core around the central mass of the galaxy?

There is no iron core around the center of any galaxy.

But: Deep inside the core of Abell 1795: the Chandra view

We present X-ray spatial and spectral analysis of the Chandra data from the central 400 kpc (H0 = 50 km/s/Mpc) of the cluster of galaxies Abell 1795. The plasma temperature rises outwards by a factor of 3, whereas the iron abundance decreases by a factor of 4. The spatial distribution of Oxygen, Neon, Sulphur, Silicon and Iron shows that supernovae Type Ia dominate the metal enrichment process of the cluster plasma within the inner 150 kpc. Resolving both the gas density and temperature in 9 radial bins, we recover the gravitational mass density profile and show that it flattens within 100 kpc as rho_DM \propto r^{-0.6} with a power law index flatter than -1 at >3 sigma level. The observed motion of the central galaxy and the presence of excesses and deficits along the North-South direction in the brightness distribution indicate that the central cluster region is not relaxed. In the absence of any non-gravitational heating source, the data from the inner ~200 kpc indicate the presence of a cooling flow with an integrated mass deposition rate of about 100 Msun/yr. Over the same cluster region, the observed rate of 74 Msun/yr is consistent with the recent XMM Reflection Grating Spectrometer limit of 150 Msun/yr.

IOW the abundence of Fe increases close to the core of this galactic cluster, probably due to enrichment from supernovae Type Ia.
P.S. 400 kiloparasec = 1,304,654 lightyears.

FYI, the term "dusty" in reference to a plasma is a reference to non ionized particles in the plasma, not a particular element (like a metal). Not all the atoms will be ionized. Heavier atoms (like that iron around the core) isn't likely to be "fully" ionized, even if it is "partially' ionized, and heavier elements will contain much more mass than a single proton.

A dusty plasma is a plasma with dust. Putting quotes around the dusty does not change the definition.
The ICM is a plasma. By definition most (99%) of the plasma is not neutral.

What is the percentage of ionized atoms to non ionized atoms? How will ionization affect the "collision frequency"? How will it effect the trajectory of interaction?

I do not know the degree of ionization of the ICM. Since a partially ionized plasma ihas an ionization of 1%, I assume that it is much less than 1% in the ICM because it is at an extremely high temperature (see below).
The degree of ionization has little effect on the "collision frequency".

What is a "trajectory of interaction"?

You're treating them exactly the same? Really? By the way, are these ions still connected to the stellar infrastructure that presumably provides the heat for ionization? Surely "neutral" atoms will not act the same as a charged ions, will they?

Yes - mean free path applies to all particles.

In physics the mean free path of a particle is the average distance covered by a particle (photon, atom or molecule) between successive impacts.

No "stellar infrastructure" is involved.
Intracluster Medium

Heating
The ICM is heated to high temperatures by the gravitational energy released by the formation of the cluster from smaller structures. Kinetic energy gained from the gravitational field is converted to thermal energy by shocks. The high temperature ensures that the elements present in the ICM are ionised. Light elements in the ICM have all the electrons removed from their nuclei.

(emphasis added)
The tiny percentage of neutral atoms (if there are any, see above) will not act the same as ionized atoms. They do not have a charge. Ionized atoms do.

 

[DazzaD]

Welcome to the conversation. Nice link. I liked the graphs and such. Here an example of how they account for "normal matter' in the BB.

Never once did they mention any of the half-life expectations of "dark matter", or explain how it is composed and how it was created or destroyed. Pure oversight or did these calculations predate any 'dark matter" discussions? How would dark matter change anything in these calculations?

Perhaps you could elaborate/explain some of the 'claims' of this link?

Perhaps if I cheekily avoid a lot of typing and recommend:

http://www.astro.princeton.edu/~dns/MAP/Bahcall/final.html

(although you have to bear in mind the "results" quoted here are 10 years old in places).

and perhaps

http://map.gsfc.nasa.gov/

where there is a summary of 'results' but also access to the technical papers.

Baryon fraction also affects the CMB power spectrum.

Indeed that was what the WMAP link was for.



When I was working on my PhD (back in the middle of the 90s) I was looking particularly at MACHOs... the overwhelming consensus is that they are unable to provide much of the "missing mass", perhaps a significant fraction of the dark matter within galaxies, but certainly not a significant fraction of the "missing mass" given the constraints mentioned in my original two links. For that we really do require a bit of a rethink and non-baryonic matter and maybe even something more radical like "dark energy" might be needed.

That isn't to say people don't work on other ideas.... blimey MOND theories have been around since the 70s but I have yet to see one that does as good a job at explaining what we see as the collection of "standard ideas" that are under discussion by most of the people in this thread.

The killer observations are the ones that say "look... I dont care what sort of combination of "ordinary" matter or forces you dream up, you aint gonna explain why the cmb is like this or why galaxies form like this or why galaxies whizz round like this".

When all is said and done, if you truly take those results to heart, they are saying that we need a more "radical" solution if you will than perhaps some people are willing to accept, and that, for me, is the real kicker here.

That scientists are willing to go out on a limb and say "do you know what, we are missing something big time here, cos all our clever stuff we know aint gonna explain this, we need to have a bit of a play around here and see if we cant work out what is going on".

i.e. scientists are doing science.

OK, fair enough, the details ARE a little hard to discuss unless you have a very sound background in the maths and physics required, and I think that is what is perhaps causing the "friction" here....

 

[Reality Check]

http://map.gsfc.nasa.gov/

where there is a summary of 'results' but also access to the technical papers.

Hi DazzaD: Thanks for the link.
I was wondering what the uncertainties on the various CMB measurements were and there they are!

3. WMAP nailed down the curvature of space to within 1% of "flat" Euclidean, improving on the precision of previous award-winning measurements by over an order of magnitude
4. The CMB became the "premier baryometer" of the universe with WMAP's precision determination that ordinary atoms (also called baryons) make up only 4.6% of the universe (to within 0.1%)
5. WMAP's complete census of the universe finds that dark matter (not made up of atoms) make up 23.3% (to within 1.3%)
6. WMAP's accuracy and precision determined that dark energy makes up 72.1% of the universe (to within 1.5%), causing the expansion rate of the universe to speed up. - "Lingering doubts about the existence of dark energy and the composition of the universe dissolved when the WMAP satellite took the most detailed picture ever of the cosmic microwave background (CMB)." - Science Magazine 2003, "Breakthrough of the Year" article

For those who are interested in cosmology, the Legacy Archive for Microwave Background Data Analysis (LAMBDA) also has the cosmological parameters derived from various models and the WMAP data (WMAP Cosmological Parameters Model/Dataset Matrix) and a WMAP parameter summary.

 

[edd]

My apologies ed. I was a little loose with the term 'you' (yet again).

Accepted, and apologies for being unnaturally grumpy myself

IMO "jumping to the conclusion" that a "new form of matter" is responsible for gamma rays from space is simply an "act of faith". It's never been seen before in our solar system to the best of our knowledge. If we observe the Fermi images however, one very obvious source of gamma rays that passes through the images is our own sun.

With all that "missing mass" to explain out there, I'm a lot more inclined to believe that we simply underestimate the number of light sources in a given galaxy due to the dust that tends to absorb a lot of the light.

I would say you had a point if it were only one line of evidence pointing at dark matter. But we have multiple lines of evidence in cosmology and all of them are pointing at the same bit of parameter space - one with a hefty dark sector.

Myself, I'm pretty sure the Fermi guys know how to account for the Sun and many other sources in the sky. Equally, I don't think we're in the position to spot an extended gamma ray source around the galactic centre and put it down to dark matter self-annihilation - I'm not going to 'jump to the conclusion' but I'm certainly ready to take a comfortable stroll towards it as further evidence comes in.

 

[edd]

Hi DazzaD: Thanks for the link.
I was wondering what the uncertainties on the various CMB measurements were and there they are!

For those who are interested in cosmology, the Legacy Archive for Microwave Background Data Analysis (LAMBDA) also has the cosmological parameters derived from various models and the WMAP data (WMAP Cosmological Parameters Model/Dataset Matrix) and a WMAP parameter summary.

To be clear, WMAP alone cannot constrain flatness or dark energy. If you assume flatness you can get the amount of dark energy. If you don't, you need to add another bit of data - the value of H_0. The WMAP-derived value of H_0 is one that assumes flatness, so that can't be used there.

 

[Dancing David]

I am glad to see that discussion is actually taking place.

 

[DazzaD]

To be clear, WMAP alone cannot constrain flatness or dark energy. If you assume flatness you can get the amount of dark energy. If you don't, you need to add another bit of data - the value of H_0. The WMAP-derived value of H_0 is one that assumes flatness, so that can't be used there.

Is that "flatness or dark energy" or is it "flatness and dark energy".

I.e..... I thought that WMAP, combined with such and such standard model set, could and has constrained the "flatness".

Or was I not paying proper attention (which is likely the case! )

 

[Michael Mozina]

That is not quite right but you are getting there. The galaxies in the colliding galactic clusters rarely collide so they do "pass through" the cluster.

Before I continue, perhaps you could answer a few questions about where you percieve the the galaxy infrastructure is located in its relationship to these "blobs". Where in your opinion is the "black hole" or "large mass object" from the galactic core, and/or the main core of stars in relationship to any of the "blobs"? How about that iron 'cloud' in relationship to the blobs?

 

[edd]

Is that "flatness or dark energy" or is it "flatness and dark energy".

I.e..... I thought that WMAP, combined with such and such standard model set, could and has constrained the "flatness".

Or was I not paying proper attention (which is likely the case! )

It constrains the flatness if you know the value of the Hubble constant or put in some other information to help.

Broadly speaking, with the CMB alone the flatness and amount of dark energy are degenerate - you can vary the two together and maintain a good fit.

The first acoustic peak in the power spectrum is basically a known physical scale, and we can measure the angular size of that physical scale in the CMB. With an angle, and the size that angle should cover we would know the distance to the CMB were it not for the fact that we don't know the geometry of the triangle unless we know if space is flat. The problem boils down to either knowing how big that triangle is or knowing the geometry of the surface that triangle is drawn on, but not being able to know both together from just an angle and one side of the triangle.

http://nedwww.ipac.caltech.edu/level5/Sept02/Reid/Reid5_2.html seems to give a good overview of the details, although its behind on experimental results.

 

[Michael Mozina]

OK, fair enough, the details ARE a little hard to discuss unless you have a very sound background in the maths and physics required, and I think that is what is perhaps causing the "friction" here....

Well, I do hear you that this is a possibility, and I've definitely had to learn their recent 'lingo" (like "magnetic reconnection rather than "circuit reconnection" or "particle reconnection"). They talk about "black holes", that are incredibly bright near the event horizon and spew jets of matter at near the speed of light. Their terminology is "confusing" to say the least. On the other hand I've been studying these topics for more than some of these folks have been on the planet, so I've grown "comfortable" with the lingo. I've also had a background in calculus, and I've certainly had to "brush up" over the last few years even to keep up with most of the papers.

The problem with the mainstream belief system is not related to the mathematics IMO. It all 'works out' at the level of mathematics. It's the "physics" (empirical) level of "reality" that their theory falls apart.

For instance:

In this scenario we have "gamma rays' and a known "natural" source of gamma rays that occur here on Earth without our help every single day, 365 days of the year. We have another known source of gamma rays in the solar system in every large planet with an atmosphere. Saturn's discharges are massive compared to the discharges in the Earth's atmosphere. Venus experiences them. Every major planet with a thick enough atmosphere is likely to experience discharges that could and do emit gamma rays. The "brightest" source of gamma rays in the entire solar system is our own sun. Fermi even "sees" our sun in these images. They too are generated by massive electrical discharge processes that the mainstream describes as 'magnetic reconnection'. "Magnetic reconnection" is something that Alfven himself referred to as 'pseudoscience' and Alfven presented these events as discharges in plasma.

This knowledge of multiple known gamma ray sources in our solar system allow us to take an Occum's razor argument to a lot of 'possibilities' that might explain "gamma rays" in any particular quantity somewhere "out there in space".

For instance: We don't need to believe that "invisible monkeys" burped out gamma rays in space "somewhere out there". It is absolutely unnecessary to invent a new "source" of gamma rays. The only variables we will need in any 'explanation' of gamma rays in space are the number of 'suns', 'planets', the voltages, and the amperage involved. That's it. We don't need any more variables to be "out there" in space somewhere.

Essentially any theory that requires more than these number of variables is introducing an "unnecessary" and "extraordinary" claim. We know of many specific light sources at these wavelengths which will exist 'out there" in space. An Occum's razor argument is likely to prevail over "any" other theory, but most certainly it will prevail over any theory that has *no* (as in zero) empirical support, and has no 'natural' source here on Earth or in this solar system.

It's never their math that's the problem. In fact I would argue that the math is the problem. They're (as a whole group) mathematicians by trade. They love math, and they are "attracted" by mathematical presentations, with or without empirical support. Somewhere however they lost their own ability to "question" their basic assumptions, even when new evidence demonstrates that they *must* reevaluate their basic assumptions. "Dark matter" could be "anything" that we know to exist in nature. It seems that "dust" can now be shown to have blocked 50% or more of the light between ourselves and various galaxies. Depending on local conditions (clouds in space), and distances involved, that dust could block a lot more light than we realize. It's entirely possible that we will need to double or triple the number of stars and planets in various galaxies just to be congruent with recent discoveries.

To sit here and claim that this "missing mass' must "necessarily' be in some exotic form of matter is unwarranted. It's "unlikely" in fact. It's a "subjective" decision by a specific individual to choose to claim we already "know with certainty' how much "normal matter" exists in any specific galaxy. Our technology simply isn't that sophisticated. We'd love to think so, but we can't even pick out individual stars in distant galaxies, let alone count all planets, etc. That "missing mass" could simply be normal material that our "guestimates" do not yet account for, but that lensing data and rotation methods do account for.

 

[Michael Mozina]

Perhaps if I cheekily avoid a lot of typing and recommend:

http://www.astro.princeton.edu/~dns/MAP/Bahcall/final.html

(although you have to bear in mind the "results" quoted here are 10 years old in places).

I peaked at them a bit last night, but I'd like to read a few of the referenced papers before I comment further. When I'm ready (several cups of coffee from now ), I'll quote the site and point out the weaknesses in the arguments. You've provided a wealth of information however, and I appreciate the links. Thanks.

I was also hoping to hear RC's explanation of where he believes the black hole and/or the core stars are located in relationship to the various "blobs" in any of the images. Perhaps you would care to comment on that question while the coffee kicks in and I read through your materials?

 

[edd]

Excuse subpar quoting and terseness. - on a phone.

Re questioning basic assumptions - that's exactly what we did to end up concluding the existence of dark matter and energy.

Re stars in distan galaxies: look up the Hubble key project. Also studies of stellar populations in other galaxies is quite well advanced.

 

[Michael Mozina]

IOW the abundence of Fe increases close to the core of this galactic cluster, probably due to enrichment from supernovae Type Ia.

So these heavier elements, are the "more likely", or "less likely" to "pass through" the process?

Does that number you quoted (9 times) also describe the amount of material you believe to be contained in what we'll call the "plasma cloud" as compared to the stars and planets, neutron stars, binary stars, black holes, etc that might be in any particular core of any particular galaxy? Aren't most galaxies expected to have "black holes' near their core?

I guess I really need to know where you believe the stars and other "MACHO" material might be located in relationship to the ICM "blobs" in these images.

 

[edd]

So these heavier elements, are the "more likely", or "less likely" to "pass through" the process?

I don't think that was RC's point. The iron density is a function of environment but that doesn't mean it has an impact on cluster mergers.

Does that number you quoted (9 times) also describe the amount of material you believe to be contained in what we'll call the "plasma cloud" as compared to the stars and planets, neutron stars, binary stars, black holes, etc that might be in any particular core of any particular galaxy? Aren't most galaxies expected to have "black holes' near their core?

Supermassive black holes are a tiny proportion of a galaxy's mass. The entire mass budget in fact is pretty well constrained for the sake of this argument.

 

[Michael Mozina]

Excuse subpar quoting and terseness. - on a phone.

Re questioning basic assumptions - that's exactly what we did to end up concluding the existence of dark matter and energy.

The part that astronomers failed to "question" is best demonstrated by your next statement IMO:

Re stars in distan galaxies: look up the Hubble key project. Also studies of stellar populations in other galaxies is quite well advanced.

IMO this is the core "belief" that astronomers failed to "question" before jumping to rather "exotic/wild" conclusions. Recent evidence suggests that a lot more light is being blocked/absorbed than we realized when we first came up with "exotic matter". IMO it's time to now reevaluate the belief that our knowledge of ordinary matter in a galaxy "quite well advanced", particularly in light of evidence from just the last few years.

http://www.galex.caltech.edu/newsroom/glx2009-04r.html
http://www.nytimes.com/2008/05/17/science/space/17univ.html

IMO it's more than likely that what you're calling "advanced" knowledge will seem rather primitive in say 100 years, and "more than dated" after just a couple of years of images from the James Webb telescope.

I think it's a "wild" and "extraordinary" claim to believe that we really have things figured out with that degree of accuracy, particularly given the complexity of the system itself. We would really need to believe we know "exactly" (within error bars and everything) how much 'ordinary matter' exists in a specific galaxy, and recent evidence suggests that we simply do not know that number with any real precision and there are complex variables to consider.

For instance, "dust" isn't distributed evenly in space. Spacetime is known to be 'threaded' based on lensing and other visual studies. There's a lot we don't quite "understand', including how many point sources might exist in any given galaxy and how much light is blocked between here and there by the interstellar and intergalactic dust. I think it's way too early to 'assume' we need exotic forms of matter/energy to explain nature. We probably/certainly need to change our way of "thinking' about how nature functions. We also need to 'question' IMO just how much we "understand' when 96% of our 'understanding" seems to be predicated on "dark" things. Where does 'dark energy' even come from?

It seems to me edd that it's time to "question" that core assumption about how advanced our understanding of point sources really is. We can't actually "count' stars in a distant galaxy, we "estimate" them and our "estimations' are full of "assumptions" that may or may not be accurate.

 

[edd]

There are certainly questions of dust and total stellar mass and while I say stellar population knowledge is advanced I am equally aware it is a hot topic of research and an area in which our knowledge lacks.

But it is not so wildly out nor so fundamental to our understanding of cosmology that it can significantly alter our estimates of Omega_b - at least that I can see. Do you really think dust can absorb so much starlight and not reemit it in other observable bands to the extent we can be out by about an order of magnitude?

 

[Michael Mozina]

There are certainly questions of dust and total stellar mass and while I say stellar population knowledge is advanced I am equally aware it is a hot topic of research and an area in which our knowledge lacks.

http://www.space.com/scienceastronomy/090608-aas-black-hole-masses.html

It actually goes beyond simply our knowledge of stellar populations, but also what goes on at the core of galaxies in general.

But it is not so wildly out nor so fundamental to our understanding of cosmology that it can significantly alter our estimates of Omega_b - at least that I can see.

It's that "at least that I can see" part that makes me nervous. There's a lot out there that we do not yet understand.

Do you really think dust can absorb so much starlight and not reemit it in other observable bands to the extent we can be out by about an order of magnitude?

I don't know. It's that 'not knowing' part that leads to my reservation about exotic materials.

For a time I entertained MOND theory (or some form of EU theory), but the lensing data actually convinces me that the material is there. I simply do not know what form it takes, but there are only so many known forms of matter. There's no point in 'making stuff up' until we've exhausted more "likely" causes from known forms of mass/energy and I don't think we are there yet, not by a long shot. Your mileage may vary.