Giant Galactic Blobs: Uncovering Dark Matter?

[Garth]

What are http://www.spitzer.caltech.edu/Media/releases/ssc2005-03/release.shtml ?

Perhaps these will give a clue as to the nature of galactic haloes and the inter-galactic medium, and perhaps identify Dark Matter as well?

Garth

 

[ohwilleke]

What are http://www.spitzer.caltech.edu/Media/releases/ssc2005-3/release.shtml ?

Perhaps these will give a clue as to the nature of galactic haloes and the inter-galactic medium, and perhaps identify Dark Matter as well?

Garth

This seems to be a bad link.

 

[matt.o]

I doubt it will identify dark matter. It seems the blobs are primarily made up of hydrogen. We already know that the inter-galactic medium in clusters of galaxies is enriched (to about 1/3rd of the solar abundance of heavier elements) by galaxies via some mechanism (eg. ram pressure stripping, supernova explosions etc), however it is generally thought that the hydrogen in the inter galaxy medium is primordial.

If you look at the links to the pictures they had, there was a small blurb that mentioned a fairly plausible idea (in my mind!) that the blobs origionate due to supernova explosions of massive star that are produced during an intense star burst, triggered by the merging event. These explosions create a 'superwind' which shoot gas outwards. here's the link:

http://www.spitzer.caltech.edu/Media/releases/ssc2005-03/ssc2005-03b.shtml

Matt

 

[Garth]

Thank you I have fixed the original link now - it is slightly different to yours Matt.

The question is do the blobs originate from the galaxies and are expelled from them, or is the blob 'primordial' and perhaps the remnant of the galaxy forming process?

If so are they still there around our Local Group, perhaps in some evolved form ? and actually the DM component is baryonic in nature?

Garth

 

[matt.o]

I don't know whether the blob is a remnant of galaxy formation. If it were, blobs would be seen around other galaxies which are not merging. From what I read in the link, it seems the blobs occur in merging galaxies, hence are probably not primordial.

 

[Nereid]

Speaking of primordial ... dwarf galaxies are sometimes thought of as being 'unevolved remnants' ... http://www.subaru.naoj.org/Pressrelease/2004/08/05/index.html suggests that at least one (Local Group) dwarf is anything but 'pristine'.

Speaking of strange, local denizens, does any PF member have comments about these http://astro.ph.unimelb.edu.au/~mdrinkwa/fornax/globulars/ ?

(note to turbo-1: they do not have discordant redshifts, are not quasars, and don't seem to be 'near' Seyferts).

 

[Chronos]

I just don't see dark matter going away anytime soon. BBN, for one, still harshly constrains how much missing matter can be baryonic. For a fairly recent paper see:

Big Bang Nucleosynthesis
http://arxiv.org/abs/astro-ph/0406663

 

[Garth]

Chronos -Yes but let the observations drive the theory and not the other way round!

We have evidence of the gravitational pull of DM but conclude that it cannot be baryonic because theory says so.

On the other hand we are observing hot gas, "giant galactic blobs", Lyman alpha forests etc. etc. all of which are baryonic; not to mention the possibility of not-yet-observed evolved, i.e. condensed, baryonic matter in such forms as black holes, 'Jupiters' and 'bricks', to mention a few.

If we add the masses of all these components, together with luminous material i.e. galaxies, we may find the overall baryonic density stretches the standard BB 4% limit.

Solution? If (a big 'if') the theory does not fit the observation - then modify the theory!
My suggestion would be to consider the 'freely coasting' scenario more seriously.

Garth

 

[matt.o]

there would have to be an absolute crapload of unobserved baryons to make up for DM!

Think of a cluster of galaxies. Observed (ie. luminous stuff) mass makes up around 20% of the total cluster mass (90% of which is the x-ray emitting intra cluster medium) . DM accounts for around 80% (roughly). A typical clusters has a mass of ~$$10^{15} M_{\odot}$$. So there has to be lots of extra (unobserved) baryons to account for!

Matt.

 

[Garth]

May it be that the 'Giant Galactic Blobs' are a new observation of part of these "unobserved baryons"?

We also have to account for the deep vacuum of the intergalactic voids, which cannot be explained by the standard model, because there has not been enough time for matter to differentiate in density under gravitational attraction. Perhaps the voids are not such a deep vacuum after all but filled with diffuse and dim unobservable ordinary matter?

Within a galaxy or galactic cluster there may be a MOND type modification to gravitation that explains the missing mass.

I think we would do well to keep an open mind.

Garth

 

[Chronos]

There is a great deal of unobserved baryonic matter - in fact, too much. I don't think this is a case of theory driving observation, quite the contrary. When you look at the observed distribution of matter and the observed motion of matter on large scales, it is hugely inconsistent [far beyond the error bars] with theory. Clearly one or the other must be incomplete. WMAP was pivotal in deciding the issue. By confirming the topology of the universe is truly indistinguishable from one that is perfectly flat, the most reasonable conclusion is we have not seen the vast majority of gravitating matter in the universe. I think it's an even bigger mistake to force theory to fit observation when observations do not conclusively rule out theory. Especially when theory is so fundamentally sound mathematically and internally consistent across multiple disciplines. Adding enough unseen matter to gravitationally balance the books is not a rash step. At this point, all the evidence suggests the majority of this unseen matter does not behave like ordinary matter aside from gravitational effects. Theory provides possible descriptions of the nature of such matter. It also predicts detection is difficult given current methods and technology. It seems reasonable to allow for the possibility theory is sound and give the observation side a chance to catch up. Theories that have otherwise proven sound should not be abandoned until thoroughly discredited by observation.

 

[Nereid]

Something I've been meaning to do since at least when Garth first posted SCC ... find a recent, good review paper on observational constraints on DM. You know,

1) here's how much DM there is in the solar neighbourhood, inside ~35 kpc in the MW, out to the MW halo, within the LG, near the LMC and SMC, near M31, near M33, near the LG dwarfs, within other nearby groups (e.g. M81), within nearby clusters (e.g. Virgo, Fornax), near the large and small galaxies in these clusters, within nearby superclusters, ... and how it is distributed

2) here are the techniques used to make these estimates, along with errors, systematics, and strengths and weaknesses (oh, and theory dependence)

3) here is a summary of the analyses which conclude that the DM in {x} has a non-baryonic component (and estimates - ranges - of what that is)

4) here are some attempts to 'make the DM go away', their domains of applicability, successes and failures, etc

While showing that within the mainstream there is some nice consistency between the 'local' and the cosmological, I feel it's also important to note that what leads to 'DM' conclusions varies a lot, depending on 'what' and 'where' e.g. no matter what cosmological model you choose, there's lots of 'local' DM, and at least a significant fraction of at least some of the local DM must be non-baryonic ... unless you tweak GR (e.g. a la Bekenstein).

 

[meteor]

We should keep in mind that dark matter is also important in the formation of large-scale structure. As many people know, there are two competing theories for the formation of structure: the Top-down scenario and the Bottom-Up Scenario. Theorists actually prefer the second: galaxies are formed of little building blocks, then galaxies form clusters, then superclusters. One of the characteristics of a universe filled with cold dark matter is that the universe develops the Bottom-up scenario... I'm not sure if this would be true with a universe filled with baryonic Dark matter...

 

[Garth]

meteorGravitationally there's no difference between baryonic and non-baryonic DM, there's just more ordinary 'stuff' to make galaxies out of if DM is baryonic.

BTW Nereid SCC is also a tweak of GR a la Bekenstein, except it only adds a scalar field rather than scalar, vector and tensor fields. A case for Ockham's razor?

Garth

 

[Nereid]

BTW Nereid SCC is also a tweak of GR a la Bekenstein, except it only adds a scalar field rather than scalar, vector and tensor fields. A case for Ockham's razor?

Er, no.

The key difference (at least so far as I've checked things out) is that Bekenstein manages to make the ugliness of the mainstream accounting for (spiral) galaxy rotations curves (and various stuff about dwarf systems too, possibly) ... by encompassing MOND in one fell swoop. AND he also manages to overcome two of MOND's biggest (consistency with good observations) weaknesses - gravitational lensing and cluster 'DM'.

I've been pressing you (maybe not hard enough) from Day1 to show that SCC makes the spiral rotation curves problem 'go away' (i.e. without 'awkward' DM profiles), AND account for the many excellent, independent observations which lead to the conclusion that there's an awful lot of 'DM' in (local) galaxy clusters.

To put it crudely, as I did earlier today wrt those who feel 'local realism' is all we need (QM? bah, 'real men' don't need that!), I'll walk out on my 'marriage with the mainstream' in a heart-beat; just show me the consistency!

 

[Garth]

Thank you Nereid that is a good point and it is 'work still in progress'.

SCC in the first instance does not make "the spiral rotation curves problem 'go away'" and is in the same position with them as the standard model. The difference being SCC identifies any DM required as baryonic. (The problem for SCC is to explain why it is not observed, which is why the Giant Galactic Blobs may be significant)

However, as an educated guess, this role of 'baryonic DM' may only hold for the cosmological DM, in order to make the matter density parameter up to 0.22 from 0.04, but not for individual galaxy rotation profiles.

The reason being that a static spherically symmetric solution of the SCC field equations has yet to be found for an extended density such as a galaxy rather than a condensed central mass such as a star. Even in that case the SCC gravitational acceleration in the Jordan frame is modified Newton:

d²r/dt² = -[1 - GM/(rc²) + ...]GM/r ²

but this does not give the MOND acceleration equation.

One intriguing thing about MOND is that although it does not explain cluster dynamics i.e. 'cluster DM', or 'cosmological DM' yet it may be connected with dynamics on a cosmological scale. The MOND anomalous acceleration a₀ = 10^-8cm.sec^-2 is roughly equal to the Hubble acceleration cH, which is the clock drift in SCC between 'atomic' and 'gravitational' (ephemeris) time.

Garth

 

[Chronos]

Garth, I respect what you have done, but I think it is wrong. The orbital decay in binary pulsars is a stake in the heart of the Jordan reference frame. I really did not understand how SCC explains this.

 

[matt.o]

what is this SCC?

 

[Garth]

matt.o SCC = A New Self Creation Cosmology.

Chronos Thank you, any gravitational theory must be vulnerable to falsification by experiment and observation, as indeed is GR.

The orbital decay of binary pulsars is a good case in point and is dealt with in my paper http://www.kluweronline.com/oasis.htm/5092775. The SCC prediction for such is the same as GR.

This is because the basis of the theory is GR - BD (Brans Dicke) modified by allowing the BD scalar field to interact with particles, as well as perturbing space-time. It interacts according to the 'Principle of Mutual Interaction' (PMI) in which: "The scalar field is a source for the matter-energy field if and only if the matter-energy field is a source for the scalar field."

Degenerate matter, such as in the Pulsar's neutron star, is highly relativistic. Its equation of state is traceless, as is that of a photon gas, and like radiation it is de-coupled from the scalar field. The matter-energy field no longer is a source for the scalar field and therefore, by the PMI, the scalar field is decoupled from a neutron star. Hence the scalar field does not interfere with the binary pulsars’ orbits. According to SCC their orbits revert to the equivalent GR orbits and consequentially orbital decay from gravitational radiation is the same in both cases.

What is different between the theories is the collapse of the progenitor stars, when they become degenerate in the first place.

In SCC G is 3/2 the measured Newtonian G, although this increased gravitational attractive force is compensated by an opposite scalar field force. However, as the stars become degenerate this latter force fades away when the scalar field decouples. Consequently the gravitational force increases to its 'naked' value and the system behaves exactly as an equivalent GR system except the forces involved are 3/2 the normal value. Consequently, using Kepler to evaluate the masses of each star results in values 3/2 too large. Also, the sudden apparent increase in gravitation would cause circular orbits to become elliptical. Therefore the observation of any pulsars with more or less circular orbits around their companions would be difficult to explain in this theory.

Furthermore, during stellar core collapse to the white dwarf stage the scalar field would gradually decouple, and the gravitational force increase to its 'naked' value, thus encouraging further collapse. Hence I conclude the Chandrasekhar limit is only 2/3 of its standard value of 1.4 M_Sun, that is only 0.93M_Sun.

So if our Sun did not lose too much mass in its red giant phase it too could become degenerate. Whether this is a feasible conclusion or not I leave up to you.

Garth

 

[Chronos]

That results is some pretty weird gravity effects during the collapsing phase. And that is not to say it is wrong. Neutrinos were basically invented to bridge the 'missing energy' gap in supernova under current theory. How does this work in a Jordan reference frame? I perceive that under SCC energy conservancy only works globally in a BD scalar field, but, how does that solution differ from GR? Furthermore, shouldn't local effects [SR solutions] should be far more obvious than is evident?

 

[Garth]

Super novae in SCC? Solving the equations of stellar structure and collapse for the SCC metric? I give up - I haven't the strength! Future work to be done if and when GPB delivers!
Basically (i.e. 'hand wavingly') in comparison with GR there should be more gravitational energy released as the core tended towards degeneracy - by a factor of 50%. But I still believe in those neutrinos!


As far as SR effects are concerned, there are no scalar field effects. However I wonder whether there should be an anisotropy of inertia because of Mach's Principle?

I don't claim to know all the answers, just that SCC is a testable alternative, I await the result.

Garth

 

[Nereid]

Garth,

Let me put to you the DM case, for 'local' galaxy clusters, and ask you to tell us how SCC obviates the need for non-baryonic DM.

1) apply the virial theorem to line-of-sight velocities of the galaxies (optical; in principle can be extended to IR, radio, and maybe X-ray one day)
2) apply GR to the graviational lenses (both weak and strong; optical - radio one day?)
3) apply gas hydrostatics to cluster X-ray emission.

These are independent methods, and yield consistent results, for the mass of the (mostly? only?) Abell rich clusters to which they've been applied to date (no exceptions?). Of course, none of these say anything about what form the mass is in!

Baryons vs all other mass:
4) detailed study of galaxies - X-ray, UV, optical, IR, microwave, radio: non-halo parts give consistent composition in terms of luminous stars, dust, gas (ionised and not). (lack of) local lensing (MACHO, OGLE, etc) tells us there is very little in the way of 'dark stars', 'rogue planets', big BH, etc (one weakness of this work is that the MW may be atypical). Rotation curves are somewhat controversial (MOND's strong suit), but whatever results they give, they are secondary. So we can estimate the baryonic component of the mass of galaxies.
5) cluster X-rays, probably the most straight-forward observations of all - intensity tells us how many electrons there are (and so nuclei). The estimated mass of the baryons generating the X-rays is considerably in excess of the baryonic mass estimated to be in galaxies, so 4) is important only to rule out galaxies as the home of the mass measured in 1), 2), & 3).
6) SZE (Sunyaev-Zel'dovich effect) - inverse Compton scattering of CMBR microwaves by IGM electrons provides an independent estimate of the number of electrons.

Again, we get consistent estimates of the baryonic mass component in clusters.

Compare the results from 1-3 with those from 4-6, and we find that baryons comprise only ~10-20% of the cluster mass!

So let's look at the various escape clauses (considering only sufficiently large amounts to make a difference; there's almost certainly some of all of these):
a) cold gas: can't be in the galaxies or between them, otherwise it'd show up as absorption lines in background quasars (etc)
b) warm gas: same as a), but would also show up as emission lines
c) dust: where is the reddening of background objects (e.g. quasars, distant galaxies, galaxies on the 'far side' of the cluster?
d) pebbles, rocks: can't be H or He (solid H rocks would evaporate); if methane, ammonia, water etc, why no strong C, N, O signature near or at the end of jets (etc)? if any other composition, where did all the metals come from??
e) planetismals, rogue planets: why no observed microlenses? (and same as d)
f) dim stars: maybe?

 

[Garth]

Thank you Nereid those are all good points.

Why cannot we see DM if it is baryonic? I don't know.
But I put my money on a distribution of planetisimals and dwarf stars that have a mass function, in which object number continues to increase inversely proportionally to mass, continuous with that of larger stars. i.e. There are an awful lot of small very dim objects out there.

In the standard model why cannot DM be identified? I don't know.
But I put my money on??...

The reason to identify DM with baryonic matter is cosmological not astrophysical. The total baryonic density is 0.2 or thereabouts, not 0.02 - 0.04 as required by standard BB nucleosynthesis. However it must also be concordant with galactic dynamics as well, as you keep rightly reminding me, so I have to leave it as an open question at present.

Thank you for your comments, they clarify for me where more work is required.

Garth

 

[matt.o]

Thank you Nereid those are all good points.

Why cannot we see DM if it is baryonic? I don't know.
But I put my money on a distribution of planetisimals and dwarf stars that have a mass function, in which object number continues to increase inversely proportionally to mass, continuous with that of larger stars. i.e. There are an awful lot of small very dim objects out there.

Garth

If this was so, wouldn't this have been confirmed in lensing searches for such items in our galaxy? I think microlensing surveys proved MACHOs makes up at most 20% of dark matter in our galaxy.

 

[Nereid]

If this was so, wouldn't this have been confirmed in lensing searches for such items in our galaxy? I think microlensing surveys proved MACHOs makes up at most 20% of dark matter in our galaxy.

Lots of wiggle room (or is it wriggle room?):

- most of the observed, (rich) cluster DM is in the IGM; MACHO, OGLE, etc only found stuff in the MW (OK, also some between us and LMC ... or was it SMC?)

- we have no real way of calibrating the MW microlensing survey results ... the observed density (IIRC) of MW dense, dim objects is way too low (if extrapolated to the whole of the local supercluster) to account for all the DM ... but so what?

- HST random field searches for dim stars (e.g. Bahcall (ref later)) didn't go anywhere near deep enough to rule these out (as point sources) for rich cluster inter-galactic space).

I think a more fruitful line of reseach (to rule out dim/black stars in the IGM) would be to look for the integrated IR of these, e.g. on a long line of sight through a few superposed clusters. Others might be (off the top of my head) 'twinkling' in the (non-core) parts of the cDs (or other ellipticals) - as signals of dim star microlensing; IGM stellar luminosity function (isolated red dwarfs have already been 'seen', as have PNe; extending this a few mag might give hints that this LF is wildly different from that in galaxies ... or not).

The aim is not to determine the prevalence of such objects, merely to show that there can't be enough of these to account for a significant fraction of the 'non-baryonic' DM in the clusters.

 

[matt.o]

the point I was trying to make was there is not even enough baryonic dark matter in the form of machos in our milky way, let alone the $$10^{15} M_{\odot}$$ or so needed to make up the dark matter indirectly observed in clusters.

 

[Nereid]

the point I was trying to make was there is not even enough baryonic dark matter in the form of machos in our milky way, let alone the $$10^{15} M_{\odot}$$ or so needed to make up the dark matter indirectly observed in clusters.

On this *we* are likely in complete accord!

However, it'd be nice to have some direct (or nearly so) observations which could rule out 'dim/dark stars' as a significant component of that cluster mass (as we can so do, for example, wrt gas, dust, or bright stars).

 

[matt.o]

yes it would be nice. it would also be nice if the dark matter problem was sorted in my lifetime. i really want to know where the answer lies!

 

[Chronos]

What really irks me is there is not enough dark matter in our solar system to even make a small planet. Then again, Kepler and Newton may have come up with some pretty weird ideas about gravity and we would be even more confused than we already are if there was a boatload of it floating around in the neighborhood.

 

[Nereid]

Thank you Nereid those are all good points.

Why cannot we see DM if it is baryonic? I don't know.
But I put my money on a distribution of planetisimals and dwarf stars that have a mass function, in which object number continues to increase inversely proportionally to mass, continuous with that of larger stars. i.e. There are an awful lot of small very dim objects out there.

OK, let's get quantitative!

Red giant stars - alone in the Virgo cluster - were reported from some HST work, in 1998.

Some http://www.journals.uchicago.edu/ApJ/journal/issues/ApJ/v533n1/39985/39985.text.html on the 'local' IMF (cf Salpeter).

Now, turning the back of the envelope in which the latest piece of junk mail arrived to something of value, what can we say about Garth's idea ("object number continues to increase inversely proportionally to mass, continuous with that of larger stars")? Extrapolating from the Virgo red giant observational data, using Salpeter (or variation), would there be enough faint stars to be within an OOM of accounting for all the cluster "DM"? If not, what sort of mass function would do the trick?

 

[Garth]

The normal IMF (Initial Mass Function) is thought to decrease with decreasing mass below about 0.5 M_Sun. However there is a caveat;
The selection effect may result in the low mass and cool objects being missed.
So the flattening off as in Nereids second link may continue into planetary gas giant sized objects.
So MACHO's may continue in abundance in the IMF below the red dwarf limit and be too small for gravitational lensing detection. Whether or not gas giants could form under these conditions may be better known when we are sure how they form under solar system conditions! (Hot or cold Jupiters?)

We do have evidence of inter-galactic gas in the form of the GGB's, the Lyman alpha forest, the material illuminated by energetic electrons in the radio lobes of active galaxies and quasars.

As I've said before its an open question for me.

Garth

 

[ohwilleke]

Something I've been meaning to do since at least when Garth first posted SCC ... find a recent, good review paper on observational constraints on DM. You know,

1) here's how much DM there is in the solar neighbourhood, inside ~35 kpc in the MW, out to the MW halo, within the LG, near the LMC and SMC, near M31, near M33, near the LG dwarfs, within other nearby groups (e.g. M81), within nearby clusters (e.g. Virgo, Fornax), near the large and small galaxies in these clusters, within nearby superclusters, ... and how it is distributed

Along this line, McGaugh (a MOND guy) has a paper describing constraints on the distribution of DM is you say MOND is wrong on DM is right. It is here:

http://arxiv.org/abs/astro-ph/0403610

Stacy S. McGaugh (University of Maryland), "The Mass Discrepancy-Acceleration Relation: Disk Mass and the Dark Matter Distribution" (Accepted for publication in the Astrophysical Journal appearing at Astrophys.J. 609 (2004) 652-666).

The mass discrepancy in disk galaxies is shown to be well correlated with acceleration, increasing systematically with decreasing acceleration below a critical scale a0 = 3700 km^2/s^2/kpc = 1.2E-10 m/s/s. For each galaxy, there is an optimal choice of stellar mass-to-light ratio which minimizes the scatter in this mass discrepancy-acceleration relation. The same mass-to-light ratios also minimize the scatter in the baryonic Tully-Fisher relation and are in excellent agreement with the expectations of stellar population synthesis. Once the disk mass is determined in this fashion, the dark matter distribution is specified. The circular velocity attributable to the dark matter can be expressed as a simple equation which depends only on the observed distribution of baryonic mass. It is a challenge to understand how this very fine-tuned coupling between mass and light comes about.

The big take home message is that even if you accept DM, it has far more observational constraints than the naiive version of DM theory would suggest. Observed DM distributions inferred from dynamical data in hundreds of galaxies are closely coupled to luminous matter distributions in those same galaxies according to a MOND like formula.

Citations to this article are shown here: http://citebase.eprints.org/cgi-bin/citations?id=oai:arXiv.org:astro-ph/0403610

Most notable is this one: http://arxiv.org/PS_cache/astro-ph/pdf/0409/0409239.pdf which looks at a lot of different fitting approaches and comes away inconclusive.

 

[Nereid]

Is anyone going to take a stab at being quantitative (re the mass function of 'stars' in inter-galactic space in rich clusters)?

 

[Garth]

Nereid a very good exercise!
OOM only. Feel free to disagree or correct my values - (thankfully) I know there is no need to say that!

Postulate: DM is (largely?) baryonic in the form of dim condensed objects, Black Holes, red dwarfs, Jupiters and smaller planetismal type bodies condensed out of primordial gas with 'Freely Coasting BBN' high metallicity, i.e. some oxygen, nitrogen and carbon so we have some ice and hydrocarbons to make 'snowballs'.

Mass of MW Halo within 100 kpc of MW centre ~ 5.5x10¹¹M_Sun

What are these MACHO's?
Some black hole/red dwarf type MACHO's have been detected, but they would account for less than 10%DM.

So assume the rest are smaller objects, how many do we need and ho dense are they?

If 'Jupiters' of 0.001M_Sun we need:
5x10¹¹/{(4/3)pi10¹⁵x10³} /psc³ = 0.1/psc³.

Now the Oort cloud is thought to be about 0.1 M_Jupiter (OOM!) so we are talking about one Oort cloud of icy bodies in every cell represented by the volume between here and Proxima Centuri, or one Jupiter every two parsecs, or one (~3M_Sun) black hole every 30 parsecs.

Problems: Not enough detected larger MACHO's, so 5% in form of (say) Black Holes, but not enough 'metals' for much ice, so we are left with Jupiters of primordial hydrogen, helium and some 'metals'.
How would such Jupiter objects gravitationally condense? I do not know, but discoveries of extra-solar planetary systems suggest there are more of them than we first thought. Jeans Mass for IGM at around 10⁰K and typical IGM densities requires galactic cluster masses. However they condensed down into smaller objects, and that process is still very obscure, perhaps most of the mass went into free Jupiters? Beyond this I am 'stabbing in the dark' and better stop!

Of course some matter would remain as gas, which is responsible for the Lyman alpha forest and the radio-emitting medium illuminated by galactic/quasar jets.
What density is required in the deep IGM to account for these?

There is another possibility. The "DM is baryonic" conclusion comes from the Freely Coasting cosmological model, perhaps there are two explanations for DM, one for cosmological (i.e. IGM DM) and another explanation for galactic DM, such as a MOND type modification to Newtonian gravity.

Garth

 

[Chronos]

I hate to go out on a limb, but could intervening matter have a gravitational shielding effect? I'm thinking along the lines of gravitational lensing. Spacetime distortion is more concentrated in-line, would it not be weaker in the 'shielded' region? Global effects would be the same, but local effects would be more intense in the focal zone, and less intense along normal geodesic lines. Would this explain the allias effect? This would be pretty easy to test since we know the mass and distance of the moon.