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Where's the dark matter? A new study finds none near the Solar System! This would be embarrassing for the popular theory, backed by good evidence from colliding galaxy clusters, that it consists of weakly interacting massive particles concentrated in roughly spherical clouds around galaxies, as shown in the artist's picture below.

But there's room for error in this new study, and I'll boldly guess it's wrong.

If you don't believe in dark matter, you should read about the Bullet Cluster:
Philip Thrift's profile photoAble Lawrence's profile photoJohn Baez's profile photoBoris Borcic's profile photo
My unscientific personal guess is that long distance gravity is a bit different from Einstein's... or anyway something different from GR and more elegant than dark matter.
What makes you guess this study is wrong +John Baez? Just intuition or you found some error/inconsistency with the analysis?
is dark matter supposed to be uniformly distributed?
+Gabriel Perren - If you read the article you can see a list of possible flaws with the study, which involves some delicate analysis. But apart from that, I can't imagine any theory that's consistent with this data and doesn't look bizarre and contrived. That's no proof of anything, but it's the reason I'd be willing to make a bet this study is wrong.
+Dee Roytenberg - The picture in this post of mine shows a hypothesized distribution of dark matter in a galaxy, and the picture in the Bullet Cluster link shows its distribution in that cluster as observed via gravitational lensing. If it's a diffuse gas of particles that only interact with normal matter very weakly except through gravity, it's hard to see how a 'holes' would form that includes the Solar System. My bet: people will try to replicate this study before getting too serious about trying to explain it.
One stupid question from me
If Dark matter does not respond in anyway other than gravity, what prevents them forming black holes. It is the interactions of particles with each other that leads to start formation instead of perpetual collapse. Why are people expecting dark matter to permeate space like the "ether" of yore and not collapse through gravity.
If and only if they have a strong repulsion between each other would they remain evenly distributed through space.

I mean, is it not ridiculous to expect a gravitationally interaction particle to pass through gravitationally massive bodies unimpaired without being gravitationally captured.

What if the excess mass of dark matter (scientists know the total quantity of known matter) be trapped in black holes
+Able Lawrence - Matter is able to collapse gravitationally only if it can emit energy somehow: energy is conserved, and if a bunch of stuff collapses its gravitational potential energy goes down, so the energy has to go somewhere.

Clouds of gas and dust that collapse to form stars emit energy in the form of light - or more generally, electromagnetic radiation. Matter that doesn't interact with electromagnetic radiation can't do this! So, you shouldn't expect dark matter to collapse and form black holes.

Anyway, people have looked for black holes as a possible explanation of the missing mass problem. Black holes can be detected by their effects, and there don't seem to be enough of them. That's one reason they postulate dark matter.
+Sergey Ten - thanks, I was actually looking for that. It's just a tiny hint: the Fermi team's own analysis of the data claimed no significant gamma ray line. But some fans of supersymmetry, desperate due to lack of evidence for this theory at the Large Hadron Collider, are already claiming it's evidence for neutralinos. Time will tell!
+John Baez I have read that without the gravitational effects of dark matter, structure formation would have been delayed in early universe and galaxies would not have formed. So they do have some collapse. In any case why should they form halos rather than being towards the center.
+John Baez But dark matter interacts with normal matter, which interacts with the EM field; perhaps it's possible for dark matter to form black holes, but it just takes it much longer to do so because it has to bleed off energy using normal matter?
Dark matter, dark energy and even MOND seem to me like some people are just trying to construct ellipses from epicycles...
It's easy to complain about modern physics, not so easy to do it better. I know: I spent the first half of my life trying.

Neutrinos, quarks, quasars, black holes... clearly nature feels no obligation to avoid surprising us. And it's actually not so terribly weird that there might be particles that interact only via the weak force and gravity, or that the cosmological constant might be nonzero - these are far less bizarre than the fact, say, that nature has decided to have 3 generations of quarks and leptons!
I place my bets on some form of a holographic gravity theory... I can't (obviously) say what kind of a theory that would be, but to me it seems very plausible that the holographic principle will play an important part. It's the only sensible explanation for the black hole information paradox!

Anyway, we live in interesting times...
In Stephen Hawking and Leonard Mlodinow's The Grand Design, they coin model-dependent realism: Model-dependent realism asserts that all we can know about "reality" consists of networks of world pictures that explain observations by connecting them by rules to concepts defined in models. <>

Then Hawking seems to go further in a lecture ""Gödel and the end of physics" <>.

So what we may left with is a changing patchwork of theories, none complete? Hawking seems happy with that.
+Grey Geek wrote: "I've always suspected that Dark Matter and Energy are attempts to prop up the Standard Model against data that doesn't conform to it."

That's a strange thing to say: dark matter of the sort most physicists contemplate can only exist if the Standard Model of particle physics is wrong!

"Matter that doesn't emit or absorb photons, making it invisible? Do we have photos showing a DM blob passing in front of normal matter and blocking the radiation it emits?"

The idea of dark matter is that it's an extremely diffuse gas of elementary particles that don't emit or absorb photons. So, it wouldn't 'block radiation' as you suggest.

I just did a calculation and it seems the usual guess for the density of dark matter in our galaxy is about 6 times 10^{-19} grams per cubic meter. So if you're imagining a big black "blob", you've got the wrong picture.

I did my calculation based on the estimate of about 0.3 or 0.4 GeV per cubic centimeter..

Maybe someone can check my calculation.

"How does DM comply with the 2nd Law if it cannot emit or absorb normal photons?"

Things don't need to emit or absorb photons to comply with the second law. Neutrinos don't emit or absorb photons. Heck, photons don't emit or absorb photons!

"What temperature is it at? How can we know?"

We don't know. The best way to measure its temperature would be to directly detect enough dark matter particles to measure their velocity distribution. A bunch of experiments are trying to directly detect dark matter particles; some have reported success but most not.

"What does it do when it is heated to plasma temperatures?"

We don't know, but it probably just gets hot and doesn't do anything exciting.

"How does it cool down? Conduction or convection?"

All these questions you're asking now also apply to a gas of neutrinos. Basically, it cools down by expanding against the force of gravity, or by transferring energy to normal matter (which however happens very slowly and inefficiently).

"Is Dark Matter made of Dark protons, neutrons and electrons?"

Almost surely not! I haven't the foggiest clue what a dark proton, neutrons and electron is. Please read Wikipedia on dark matter:

"In the bullet cluster video the "real" matter seems to be dragged around on a leash by the Dark Matter. Why isn't it accelerating toward the Dark Matter, to eventually collide and absorb or be absorbed?"

I haven't seen this video. What video?
By the way, +Grey Geek and everyone else: I like to answer physics questions, but please ask just one at a time, instead of issuing a barrage of them. It takes about 5-10 times longer to answer this sort of question than to ask it. Also, the answer to one question will often help you improve the next question.
Has anyone started to include the 'room temperature' stars, several of which are near (<200LY) from here? Their hitherto unknown (perhaps 'dark' would be a good word) mass does need to be included.
Other than the initial announcement of the finding of a few cold stars near us, I haven't heard much about this. Anyone else hear anything?
We assume gravity to follow inverse square law at all scales. Interactions between quarks behave strangely with distance with attractive force peaking at a certain distance and then dropping off nearer as well as farther.
What if gravity differs at extremely low scales (closer to Planck length) and rises at galactic scales (if you consider galaxy as analogous to a nucleus) things would revolutionise physics and explain stuff.
If you consider multiverse theory, and one of the possibility being considered to explain the anthropic priciple question is evolution and natural selection
The earliest evolutionary pressure on the strength of gravity is formation of galaxies.
Gravity has to be strong enough to form galaxies.
Unfortunately we have no way to study gravity at Planck scales except inside black holes perhaps and we cannot really study gravity itself at galactic scales precisely without indelendently knowing mass itself and we estimate mass from gravity
+Grey Geek wrote: "my question assumes that DM is similar to real matter not sub-atomic particles."

I'm not sure what 'real matter' is, but maybe you mean atoms and molecules: all these are made of sub-atomic particles. The currently most widely believed theory of cold dark matter is that it's made of elementary particles of a sort that don't interact via the electromagnetic force. Any other explanation runs into a lot more difficulties.

"I never got the impression that it's distribution in space is as thin as 6X10^{-19} g/m^3."

Yes, that's all you need to explain the fact that galaxies rotate much more rapidly than they would if all their mass was in forms of matter we can see or otherwise understand.

"I had read the Wikipedia article on DM, and came away with the impression that most of the claims about it are pure speculation, and have little hard data to back them up."

Actually, the article lists a lot of experimental evidence supporting the existence of dark matter, including: 1) the speed at which galaxies rotate, 2) the speeds at which galaxies move around in galactic clusters, which is again faster than compatible with the gravitational potential well due to the observed matter in these clusters, 3) the amount which galaxies clusters bend light due to their gravitational lensing, which is again too much to be explained by the visible matter, 4) gravitational lensing in the Bullet Cluster and at least one other pair of colliding clusters, which shows that while the visible matter has stopped dead in its tracks, most of the mass is continuing to move, 5) the cosmic microwave background, which shows more 'lumpiness' in the early universe than we can explain by the gravitational clumping of visible matter, 6) the redshifts of distant supernovae, which show that the universe is not expanding in a way consistent with general relativity unless we posit dark matter and dark energy, a separate phenomenon, 7) similar redshift measurements of the spectral lines of hydrogen in distant galaxies, 8) studies of galaxies in the very early universe, which show that they formed faster than we can explain by the gravitational attraction of visible matter.

All these measurements produce numerical data that seems compatible with a certain density of fairly cold dark matter, and very hard to explain otherwise. It's easy to explain one or two of these anomalies using another theory - but that's not good enough. One really needs to explain all 8.

This is why most physicists believe in dark matter. We're not whimsical romantics who leapt rashly into positing a new form of matter: we were pushed into believing in it over decades, by dozens and dozens of experiments.
+Grey Geek wrote: "I just read the abstract of this paper published on 4/11/12: The dark matter crisis: falsification of the current standard model of cosmology."

Okay, now I get what you meant. There are two completely things sometimes called the 'standard model'.

1) There's the Standard Model, which is a model of particle physics and all known forces except gravity:

The most popular theory of dark matter says this Standard Model is false: that dark matter consists of the elementary particles that are not in the Standard Model.

2) There's the ΛCDM Model, sometimes called the standard model of big bang cosmology:

This model includes dark energy (that's the Λ) and cold dark matter (that's the CDM).

Either way, saying that dark matter and dark energy are invoked to "prop up the Standard Model" sounds strange, since dark matter contradicts the Standard Model of particle physics, while dark matter and dark energy are fundamental tenets of the ΛCDM Model.

The fact that the Standard Model and ΛCDM Model are in contradiction simply means that either 1) we have more to learn about particle physics or 2) we are seriously confused about cosmology and astrophysics. That's okay: we're not done yet.
+Grey Geek - "But, one has to wonder ... if a particle of DM is larger than a photon and it is in the flight path of the photon, but cannot react with it, what happens?"

I wouldn't talk about the 'size' of an elementary particle: they don't have a nonzero size as far as we know. A more relevant length scale is their wavelength. But in any event, it's very easy to say what happens when a photon meets a neutrino, or a hypothetical dark matter particle: nothing! It just sails merrily along.

Photons don't interact with other particles because they're big: they interact with them if, and only if, those other particles have electric charge.

By the way: a lot of the questions you're asking about dark matter apply equally well to neutrinos, and we've studied those much more, so then we can use not only theory but also experiment to say what happens in all sorts of situations.
+John Baez I am a bit surprised to see you relate this piece of news to the Bullet Cluster "observation" of DM, but not to the contrary Train Wreck Cluster "observation". Should this be read as the suggestion of the latter having (in contrast to the former) among physicists a status similar to what you claim for this new study, eg doubtful and in wait of confirmation ?
+Boris Borcic - I just know what "all physicists know" (or should know) about cosmology; I haven't even heard of the Train Wreck Cluster. So, time for me to study some more, and see what people think about it.

(I'm not trying to argue for the existence of dark matter, just explain a bit about what physicists know about it, so nonphysicists can realize that it's not a nutty or whimsical idea.)
+Boris Borcic - okay, I did a little quick reading, and it doesn't seem that the Train Wreck Cluster data is 'contrary' to the Bullet Cluster data when it comes to what we're talking about now, namely the existence of dark matter. In fact it seems to strikingly confirm that! It would be very hard for one of the alternative theories, like MOND, to explain a huge region of seemingly empty space that exerts a gravitational pull! But it does raise interesting new puzzles.
+Able Lawrence wrote: "What if gravity differs at extremely low scales (closer to Planck length) and rises at galactic scales (if you consider galaxy as analogous to a nucleus) things would revolutionise physics and explain stuff."

Gravity is not just Newton's inverse square force law anymore: gravity is described by general relativity, and we need general relativity to explain many features of our universe. We can try to tweak general relativity so that gravity acts stronger at large distances and perhaps avoid the need for dark matter. Of course lots of people have tried this - it's an obvious idea. But it's very hard to get it to work! The best attempt so far is MOND, which I mentioned earlier:

But it's an inelegant theory, and it has trouble explaining things like the Bullet Cluster and (even more so) the Train Wreck Cluster.
+John Baez I really just meant to say that Abel 520 has in common with the featured study to do brutality to expectations on DM that the Bullet Cluster comforted.

My use of quotes around "observation" wasn't so much meant to deny DM than as a reminder that can't count as direct observation according to the common sense of the word the deduction of DM distribution from astronomical images; or IOW as a reminder that "observation of dark matter" is practically an oxymoron. This could probably be put in terms of the global/local character of the involved image processing.

I must add I find quite fascinating the optics of DM together with Einstein's direct paternity of it, what turns DM into a observational/experimental transposition in the largest of scales, of the well-advertised theoretical resistance of general relativity to unification with quantum mechanics.
In particular, assuming well-founded the path chosen by string theories to solve the theoretical problem with assuming supplementary dimensions, I can't help seeing in the above a hint that might be ill-founded the further idea that these supplementary dimensions should all be hidden to our perceptions by the smallest rather than by the largest of scales.
The articles mention that one of the ten assumptions in the study was that all the stars near the suns neighbourhood were moving at the same speed around the galactic center!!!
Since dark matter is primarily used to explain how starts closer and farther from the center move at the same speed but does not get ejected out; this assumption may be indirectly introducing dark matter effects
A fun thing with wording extreme presumptions or hypotheses is how it brings to mind slightly less extreme but very different hypotheses I could make for what I know. So : why couldn't DM be formed of primeval massive neutrinos released early and cooled down "close to freezing point" by the end of hyperinflation ?
Hum, this brings to a narrative that would enthuse me coming from a hard SF author :

The inflationary phase of the Big Bang stopped cold by the Pauli exclusion of massive neutrinos. A transition point at which the Universe was completely filled with a neutrino version of degenerate matter... a single huge mono-crystal of neutrinos - bouncing the Universe off breakneck inflation, and I guess then breaking up, but at the comparative snail's pace of Hubble expansion.
+Boris Borcic wrote: "So: why couldn't DM be formed of primeval massive neutrinos released early and cooled down "close to freezing point" by the end of hyperinflation?"

It could be... but I believe these neutrinos would have to be a lot heavier than the 3 kinds of neutrinos we know - electron, mu and tau neutrinos - to do the job. People have studied and rejected the possibility that the neutrinos we know could serve as dark matter. Lots of them were created in the early universe, and they decoupled from ordinary matter about 1 second after the Big Bang when the density dropped low enough. But they're not heavy and (thus) slow-moving enough to form galaxy-sized 'clumps' due to gravitational attraction.

A very heavy species of neutrino could do the job. But a more fashionable guess is a 'neutralino':

This guess is fashionable because it's a generic prediction of supersymmetric theories.
Thanks. Btw got a recent preprint googling "degenerate fermi gas of neutrinos", eg "Dark Neutrinos" by A. Nicolaidis at Can't really judge quality, but the author appears to opt for a sterile neutrino at 10eV.

And wikipedia on the matter refers to ^ Th. M. Nieuwenhuizen (2009). "Do non-relativistic neutrinos constitute the dark matter?". Europhysics Letters 86 (5): 59001, available as The paper analyses the Abell 1689 cluster, and suggests half of DM could be neutrinos. A fun figure is a Mly scale of the quantum behavior.
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