Most Sensitive Detector Yet Fails To Find Any Signs of Dark Matter

ananyo writes "A U.S. team that claims to have built the world's most sensitive dark matter detector has completed its first data run without seeing any sign of the stuff. In a webcast presentation today at the Sanford Underground Laboratory in Lead, South Dakota, physicists working on the Large Underground Xenon (LUX) experiment said they had seen nothing statistically compelling in 110 days of data-taking. 'We find absolutely no events consistent with any kind of dark matter,' says LUX co-spokesman Rick Gaitskell, a physicist at Brown University in Providence, Rhode Island. Physicists know from astronomical observations that 85% of the Universe's matter is dark, making itself known only through its gravitational pull on conventional matter. Some think it may also engage in weak but detectable collisions with ordinary matter, and several direct detection experiments have reported tantalizing hints of these candidate dark matter particles, known as WIMPs (Weakly Interacting Massive Particles). Gaitskell says that it is now overwhelmingly likely that earlier sightings were statistical fluctuations. Despite the no-shows at XENON-100 and LUX, Laura Baudis, a physicist on XENON-100 at the University of Zurich in Switzerland, says physicists are not ready to give up on the idea of detecting WIMPs. They may simply have a lower mass, or may be more weakly interacting than originally hoped. 'We have some way to go,' she says."

Comments

First dark matter post

[GameboyRMH]

Pulling in other posts below it :D

Re:First dark matter post

[The_Wilschon]

Weak.

Maybe

[Anonymous Coward]

Maybe it's just not there.
Dark matter always reminds of the 18th century hypothesis of the aether.

http://en.m.wikipedia.org/wiki/Luminiferous_aether

Same principle. Same made up matter that no one can see or detect but somehow fills the entire universe.

Re:Maybe

[TheCarp]

Yes but I could pick another example, the nutrino and say it sounds like that too:
"In 1930 Wolfgang Pauli proposed a solution to the missing energy in nuclear beta decays, namely that it was carried by a neutral particle " ( http://www.ps.uci.edu/physics/news/nuexpt.html )

It makes perfect sense. You have theories that test to a high confidence in every way you can test them, then you find an anomaly in specific instances. Whats the response? Take those theories and attempt to narrow down the properties of what would cause the anomaly.

It obviously doesn't always produce a hypothesis that pans out as correct, but, can you really say that Aether theory was so bad? It was wrong, yes, but, it lead to the creation of experiments that answered new questions and ultimately, shaped the theories that came after it.

and...at the time... that is, after light was shown to be wave-like AND before we knew that there was no motion relative to its "medium", postulating Aether made a lot of sense.

Re:Maybe

[lgw]

We can certainly detect dark matter. The CMBR studies have show it fairly directly (we've "observed" dark matter as much as we "observe" things with an electron microscope or radio telescope). The ratio of "normal" matter to "dark" matter in the early universe has been measured to 2 significant digits (perhaps more since last I looked into it).

The unknown part is what dark matter is made of. We know it's there, we just don't know what it is.

Re:Maybe

[Anon-Admin]

What we have is a phenomenon that is not explained by the calculated mass of the universe. As a filler we have titled it "Dark Matter" and "Dark Energy" and given it a mathematical correction to the calculations.

The mass issue is fixed if we realize that the size of the universe is larger than the visible horizon. Meaning it is bigger than we can see. With that we can assume that we can only see 13% of the whole universe and that the reset of it is too far away to see. Now, run those numbers through the formula to calculate the expansion rate of the universe and you get some great results!

The energy issue disappears when you realize that the closer an object is to a gravity well the slower time moves. Thus there is a large time differential between the edge of a given galaxy and intergalactic space. This time differential accounts for the perceived added gravity.

Better yet, paint it hot pink and put an SEP field around it. It is a better solution.

Re:Maybe

[FatdogHaiku]

Or there could be some interference.
From Security Video:

Jedi WIMP: These aren't the WIMPs you were looking for!
Physicist: "These aren't the WIMPs we are looking for!"
Jedi WIMP:They can go about their business.
Physicist: "They can go about their business."
Jedi WIMP: Move along.
Physicist: "Move along... move along."
Physicist: "Damn, we still haven't found anything!"

Re:Maybe

[icebike]

True, even failed theories advance science in some way or other.
However at some point you have to let them go.

The summary where it clearly states:

Physicists know from astronomical observations that 85% of the Universe's matter is dark,

I suggest they KNOW no such thing, and merely postulate dark matter to get their equations to balance. But how many such equation balancing inventions are laying in the dustbin of Physicists' revised theories over the years?

Unless or until the Physicists can find fault with the detectors, all of which have failed to find a trace of something allegedly composing 85% of the universe , it would seem that the whole "dark matter is known to exist" statement needs to taken down a notch. Detectors designed to their own specs fail to produce a single trace. It doesn't matter that there are very precise measurements of exactly how much the equations are out of balance.

Re:Maybe

[Sique]

That's what they are doing with the experiment. They know that there is a difference between the observed gravitation inside the galaxy and the expected gravitation from the visible matter. They know a lot of properties the missing matter has not: it doesn't interact with anything else than gravitation. Thus it does not interact for instance with the electromagnetic force, it is thus electrically neutral. It has no magnetic spin. It does not absorb photons. It does not interact with visible matter except by gravitational force.

This experiment tries to find some other interactions, but none so far were detected.

Re:Maybe

[Valdrax]

At what point did it become ok in the scientific community to keep on with a theory that evidence contradicts?

Where has it been contradicted here? The failure to observe WIMPs by this experiment doesn't mean that they don't exist -- just that they don't have certain properties that would make them detectable by this instrument.

It's like the search for the Higgs boson. There were theories that allowed for the Higgs to exist at lower energy levels than it was eventually found at. We tested them with the LEP and with Tevatron, in the 1990s. As we ruled out those lower (and some higher) energy levels, we got closer and closer to the truth. The Higgs boson exists are a mass somewhere around 125 GeV/c^2.

All this experiment has done is narrow the parameters a bit so far. Did you make a similar cry in 2011, when Tevatron shut down that we shouldn't have been wasting money on the LHC because the Higgs was contradicted? If so, then shame on you then. If not, then shame on you now.

The day I realized that the previous three chapters I had read were not science, but rather theories that were based on other theories based on yet other theories that only existed because the first theory was shown to be wrong at some point, was a real downer.

How is that not science? Science is all about filling in the gaps and trying to find explanations for what we don't know -- including the things we didn't previously know we didn't know. It's not some divine revelation that you either get right the first time or you disregard it as heresy and falsehood. It's a global learning process.

Re:Maybe

[icebike]

We can certainly detect dark matter.

No, we can't.
We only know to what extent our speculation and our math fails to completely work to our satisfaction.
Se we invent a black-box term to get the math to work out. We are quite precise in our invention.
We design instruments to detect this stuff that the math predicts is there. Instruments fail, time after
time.

You always need to consider the fact that it might be something else in the math that is wrong.
Otherwise, you might just as well attribute it to unicorns.

Re:Maybe

[Anonymous Coward]

Protip... that still doesn't explain the rotation curve problem observed in spiral galaxies.

Re:Maybe

[boristhespider]

"What we have is a phenomenon that is not explained by the calculated mass of the universe."

Vague statement. What we have are two phenomena, one which is not explained by the observed mass in galaxies or in clusters, and one not explained by the present (and currently only serious) model of the universe. Feel free to propose alternative models for the universe... but make sure that they fit the current observations *at least* as well as that model and fails to break the Solar System. That is hard to do.

"As a filler we have titled it "Dark Matter" and "Dark Energy" and given it a mathematical correction to the calculations."

True, with the correction above.

"The mass issue is fixed if we realize that the size of the universe is larger than the visible horizon."

No it isn't. That will do precisely nothing for the rotation curves of galaxies and will also basically do nothing for the cosmological problem either. Vague hand-waving and appeals to Mach's principle don't hold without a concrete model. Provide that model and people may be convinced, but at the minute what you're suggesting is startlingly acausal and, as a result, unacceptable.

"Meaning it is bigger than we can see."

Very true. No-one thinks that the entire universe is the observed universe.

"With that we can assume that we can only see 13% of the whole universe and that the reset of it is too far away to see. Now, run those numbers through the formula to calculate the expansion rate of the universe and you get some great results!"

Nope, you get precisely the same results that we currently get, because while it may startle you, that's what we currently do -- effectively. Thanks to causality, matter outside of our horizon cannot have an effect on us. Basically, something which is far enough away from us that light cannot have made the distance cannot possibly have influenced us. That, or you have to propose a new theory of gravity -- good luck with that one. It's a common game in cosmology, and one which precious few people since Einstein have had any luck at.

"The energy issue disappears when you realize that the closer an object is to a gravity well the slower time moves."

No it doesn't. Do you think that we're using non-relativistic models of cosmology? Relativity is at the heart of your statement that gravity wells dilate time, and relativity is at the heart of cosmological models.

"Thus there is a large time differential between the edge of a given galaxy and intergalactic space. This time differential accounts for the perceived added gravity."

Now this is a much more interesting statement. Dig out Wiltshire's attempts to use time dilations between galactic clusters and voids to explain the dark energy problem, firmly in the context of general relativity. The fundamentals are not well-studied, but it is promising. However, it goes the opposite direction from your surmise -- it tends towards providing a dark energy rather than a dark matter. It does drive home the point though that it is vital to actually try and calculate something based on an idea, properly rooted in a concrete theory. The answers might be rather different from what you expected...

Re:Maybe

[occasional_dabbler]

One of the most enlightening books I ever read was Peter Woit's crticicism of string theory The problem with modern physics is that it now takes so long to learn what has gone before that you are past your productivity peak by the time you have the tools needed to be able to contribute. Put very simply - mankind is close to the limit of what we can work out. We need either a genius way further out on the curve than Einstein or Hawkins (who doesn't want to just become an investment banker...) or we need an extrordinarily lucky break. We won't be getting better data than the LHC has provided for another century,

Re:Maybe

[Immerman]

I believe a solution to the rotation curve problem has actually been proposed by analyzing galactic motion using General Relativity-based gravity equations rather than the much simpler Newtonian ones. Using the more accurate equations renders an expected rotation profile far more consistent consistent with observations to within a tiny percentage.

Of course that doesn't explain some of the other phenomena that supports Dark Matter, but it could mean we're looking for it in the wrong conceptual places.

Re:Maybe

[boristhespider]

I'm a professional cosmologist, and I have to take issue with your first statement. The instruments did not, and categorically have not, detected the presence of something that is matter. If they had, that would be a direct detection of dark matter, and a Nobel prize would already be sitting on their desk. What they have detected are indirect signals of dark matter. It is very hard to reproduce the observations - particularly the cosmological observations - without adding at least one component of dark matter. So the observations are typically interpreted in terms of dark matter.

But this is very much not, strictly speaking, necessary. What we have is something that has an effect which, when viewed through a Robertson-Walker model, looks for all the world like a species of massive, weakly-interacting particle (or two or three such species - no-one ever said there has to be only one). On smaller scales, we have what for all the world appears to be a large amount of mass that can't be seen.

Any of this could be down to a modification of gravity. We know the nature of gravity roughly up to the position of the Voyager craft -- call it 300AU to be generous. We are extrapolating that a thousand times to get to galactic scales, a million times to get to cluster scales, and a thousand million times to get to cosmological scales, all without evidence. Of course, without a better theory to replace relativity, it's the best we can do, so we do it - but don't try and claim that instruments have detected that it is matter (they haven't), nor that we are wedded to particulate dark matter (with caveats, we aren't; the caveats are firstly that neutrinos have a mass and are therefore a rather warm dark matter, and secondly that it seems rather unlikely that there isn't at least one species of weakly interacting matter which would act as CDM, but maybe not in sufficient abundance to answer our woes).

Re:Maybe

[boristhespider]

It's always enlightening to see how it looks to people who have had occasional glimpses from the outside but never bothered looking any further.

No-one is so wedded, philosophically, to the idea of CDM as is. Everyone knows its an approximation. The arguments over what it *is*. Mirage, particle, multiple particles, modifications to gravity, unanticipated effects of relativity on large scales, unanticipated effects of *averaging* observations across large scales, or a combination of the lot of them. And I can guarantee that practically no-one has been arrogant enough to stand up in a room and declare that we know what dark matter is.

I saw one person - who shall remain nameless - say something along these lines. He said to a room full of distinguished cosmologists (and me, I'm not distinguished at all), and I paraphrase since this was a few years back, "We can be absolutely certain that supersymmetry exists". That quite took my breath away. Firstly: no we can't be. Secondly: lol. Thirdly: winning that prize obviously turned you into an even bigger prick than you already were. I can't remember if anyone made these points to him because his talk was so stultifyingly boring, and so overlong, that I was comatose long before the end. Anyway, the corollary of his flabbergastingly inaccurate statement is that he also believes firmly that there is a single species of particulate dark matter, since this is more or less a prediction of general supersymmetric theories.

He's wrong, anyway. There may very well be supersymmetry, but we can in no way be certain that it exists.

Same goes for "dark matter", whatever you want to call it. The only thing you can't do is deny that the problem is there, and that the simplest explanation, which basically works all the way from galactic scales up to cosmological scales, is that it is composed of massive, weakly-interacting particles.

Re:Maybe

[boristhespider]

The difference here is that whereas normally the "indirect" signals we receive are photons directly from a particle, or indeed a measurable and reproducible influence on known quantities in a laboratory setting (which includes the tracks of known particles through accelerators), dark matter is not easily amenable to such tests. We only see it (interpreting "it" loosely -- the way I use the words, 'dark matter' should be interpreted as 'the fact that galaxies, clusters and the universe as a whole act as though there is more matter than we observe', which is probably infuriatingly vague :( ) through its gravitational effects, and by the sheer weakness of gravity and the impractical idea of creating, well, galaxies in a laboratory setting it is never going to be directly detectable that way.

The Higgs boson, on the other hand, was seen in reproducible experiments. I do agree that we can quibble on whether it was a direct detection, or whether it was indirect, given that its existence was ultimately deduced from the pattern of particles around it - but there are big differences. For one thing, a (relatively) quick analysis of the shrapnel from a collision that produced a Higgs will point to a particle of a particular mass and nature. That can then be reproduced (albeit at a low likelihood, given the nature of the experiment), and has been. We only even saw announcements from CERN when two independent experiments both reported an excess at the same mass. (In particle physics these certainly used to be called "resonances" -- when you find that collisions with a particular energy change nature dramatically, you can be pretty certain there's a particle there. For all I know, they're still called resonances, but my particle physics is second-hand through textbooks and therefore about 25 or 30 years out of date.)

It basically comes down to a detection on local scales, under conditions we can control, through a force other than gravity. We can't examine anything through gravity - it's uselessly weak, and impossible to control. That's a "direct detection", and can be through interactions with photons, or the influence of the new particle on the particles we observe coming out of its interactions and annihilations, or anything along those lines that can be seen, influenced, reproduced, observed. We can't do that with the evidence for dark matter. All we have is that galaxies rotate faster than they should (and they do, unequivocably), and that clusters should not really be bound (but they are, equally unequivocably), and that we cannot account for this with our current theories of gravity. The easiest solution is at least one particulate dark matter, certainly -- but if that exists it *is* amenable to production in a lab, even if to actually observe it we would have to wade through ten times more data than the LHC pours out, or a billion times more. But that isn't the only solution, because the only evidence we have is through gravity, and there is absolutely no reason at all (and it would be a mild form of intellectual blindeness) to prematurely declare that "dark matter" is definitely particulate and not, say, a sign that gravity does not behave on kpc scales the way it does on AU scales, let alone on Mpc and Gpc.

Re:Maybe

[boristhespider]

Yeah I tried to go through some of that stuff years back, and it was distinctly unconvincing, sketchily-laid out, and in a far weaker state than the author(s) would wish you to believe. Ultimately, if they feel they have a truly viable theory they have to apply it, in as much detail as the current LCDM model has been applied. That means they have to start off in the early universe (or the distant past, if you prefer; we don't *have* to assume a Big Bang), then justify in some way the existence of both the cosmic microwave background, and the exact spectrum of perturbations on it; then in the same, self-consistent coherent model, they have to account for structure formation and the presence of a wave imprinted on the largest scales of galactic structure which just happens to have a wavelength that perfectly matches that on the CMB... if the universe evolved as predicted by a Lambda CDM model; they have to include a form of nucleosynthesis to explain the ratio of elements we see in the oldest stars; they have to explain why old stars tend to be metal poor and young stars are metal rich; they have to explain the collapse of shards in clusters to form galaxies; and so on and so on.

Do that, and people might just start paying attention... but they have to do it at a level of rigour that is equivalent to that employed in professional cosmology. If they can't, they don't have a theory, they have words, and words are extremely cheap. It has to be couched in a mathematical language, and that's because it has to have a surmise and make a testable prediction. It has to be directly testable. I am very definitely not a fan of Lambda CDM, and a hunt back through my posts on /. that relate to cosmology would probably make that quite clear, but I've spent many years looking at it and its perturbations anyway. In my view, Lambda CDM has one absolute killer of a prediction: the wavelength which it predicted, from that on the CMB, was imprinted on the large-scale structure, and which was later found, exactly where it said. That wavelength, and the amplitude of the wave, is exquisitely sensitive to any change in the evolution of the perturbations, which is itself exquisitely sensitive to a change in the background spacetime. Lambda CDM got it right; any successor model -- and I hope to God there is one, because Lambda CDM is not satisfactory -- also has to.

The last that I knew, the Electric Universe stuff doesn't do any of this. (I would emphasise again that to gain acceptance it is not enough to posit a model -- and it's not even enough to present some back-of-the-envelope calculations. Frankly, the absolute minimum is a full analysis of possible backgrounds -- containing at least photons, neutrinos and standard model matter -- before you can even think of putting a paper out. That would then need to be followed up with an analysis of the perturbations, which we are all after all made from. Effectively, a version of the CAMB code, or one of its competitors, is necessary. Without it, you don't really have a viable model, just yet another model that can recreate something with observables matching the background Lambda CDM, and those come ten a penny. And so on. This is not an easy job, which is why we have no answers yet -- but it sure as shit isn't because the people working in the field are purblind idiots devoid of imagination or soul. Well, certainly not all of them ;) )

Re:Maybe

[Kevin Fishburne]

I suspect sometime this century the combination of creative human genius (throwing ideas at the wall) and extraordinary increases in computational power and AI capability will take away much of the burden you describe. For example, a physicist (or even an amateur) could state a hypothesis in plain language to the AI which would then parse its meaning, request clarification if necessary, restate the hypothesis in more specific terms for verification, then attempt to adjust known theories, algorithms, laws, etc., to see if the observed data set more closely matches and report how close that match is. Basically take what people are good at (being creative) and what computers are good at (doing what they're told) and try to marry them to science's benefit.

Re:Maybe

[IndustrialComplex]

So basically you're using the same logic people use to justify the existence of God? How very..... scientific of you

No...

We have observed 'Y'. We think that 'X' might be what is causing 'Y'. We setup an experiment to test for 'X' The experiment did not detect 'X'. The observed 'Y' still exists, but we now know it is not caused by 'X'.

Or an example:

Every morning, my newspaper is delivered. I think that it is being delivered by car. I have a special 'newspaper delivery car' detector. I setup the detector, and check the results the next morning. The detector did not detect any 'newspaper delivery cars'. The newspaper was still delivered, but I now know it was not delivered by car.

Y = Newspaper delivery
X = Delivery by car
Experiment = Check for delivery cars
Result = proof that delivery was not by car

Yet we know 'Y', the newspaper delivery, occurred/exists even though we have eliminated one of the ways in which it could be occurring.

Re:Maybe

[boristhespider]

Yes, in a sense. I have a strong suspicion that if we were able to do a proper statistical mechanical analysis of the situation we'd see some odd emergent behaviour -- a galaxy is, after all, a rarified gas of about 10^9 interacting, confined bodies. We'd get different behaviour in a cluster, and different on cosmological scales.

Of course, I may be wrong and what we'd get out would be effectively pressureless dust, which is what we currently put in. Thep roblem is that at the minute we can't do a proper statistical mechanical analysis. We don't even have a full theory to work with, though there's progress here, too.

Re:Maybe

[cavebison]

> The mass issue is fixed if we realize that the size of the universe is larger than the visible horizon.

Sigh, completely wrong.

1. Dark Matter (or some kind of "unseen gravity source") has to be present *within each galaxy* to stop galaxies flying apart because of their spin, which calculations based on their visible matter says they should do.

2. Dark Matter (or some kind of "unseen gravity source") has to be present between us and certain distant objects, because of the visible effect of "gravitational lensing" (ie. visible distortion of light) being caused by something we can't see.

There may be other examples of why DM is a thing, but those are the main two that pop to mind.

TL;DR it's a LOT more than just "adding up" the required matter in the universe.

Physicists know

[Spy Handler]

from astronomical observations that 85% of the Universe's matter is dark"

They don't *know*, they're deducing this from reconciling observed data with general relativity but it's far from certain.

However relativity is not infallible, maybe it's true only in a special case -- like how Newtonian mechanics works great but only in a special case (bigger size than quantum scale, less velocity than ~1/10 c, etc)

Maybe at very large size and mass such as galaxies, general relativity doesn't hold and there's a better theory for explaining motion and gravity. If so we wouldn't have to invent nonexistent dark matter to account for the faster-than-expected galactic rotation and other things.

Re:Have they considiered...

[wonkey_monkey]

Hur hur, yeah, stupid scientists with their "degrees" and their "experiments."

What a bunch of losers.

Re:Have they considiered...

[invid]

It's there. We've detected it from its gravity. They were just hoping that it wasn't completely dark. It's starting to look like it is. The trouble with it being completely dark is that would make it difficult to prove any theories about it. What they're doing is searching for their keys under the streetlight when they've probably fallen down the sewer.

Direct dark matter detection is confusing

[amaurea]

Several different experiments have tried to measure dark matter directly in the lab, and the experimental situation is pretty confusing. This plot shows the confidence intervals and exclusion limits for various experiments (but it does not include LUX yet). The shaded regions are confidence intervals, that basically say "we've seen dark matter, and its properties lie somewhere in this region. But the dotted lines say "we haven't seen it, and if it exists, it can't lie above these lines".

What is strange, then, is that all of the detections are in regions that have been excluded by other experiements. LUX just makes the situation even more strained by pulling those upper bounds even lower. Still, those bounds and intervals depend on assumptions about the properties of dark matter, and it may be possible to reconcile the results.

It will be interesting to see what happens to those tentative detections when they get more data. My bet is that in the end some systematic effect will be found to be responsible for the apparent signal. Or (much less likely) that they were just flukes. But who knows?

Re:Have they considiered...

[tylersoze]

Guess they should have given up on the Higgs boson search 10 years ago, too? A negative results is not a "failure", it just constrains things a little more.

The most compelling evidence for dark matter is http://en.wikipedia.org/wiki/Bullet_Cluster

Obviously we should always be open to alternate hypotheses, but at the moment dark matter is still the most straightforward explanation.

Re:Dark matter fighting dark energy

[Zalbik]

It looks like bad science when they keep fiddling with the numbers to patch up their deficient theories.

Or to put it another way:
1. Scientists come up with theories to explain a phenomenon
2. Test to confirm
3. New observation breaks the theory
4. Theory refined to account for new measurements
5. Goto 2

That doesn't look like bad science at all.

The dark matter thing is stuck at step 2 as it may be either (a) the theory is wrong or (b) dark matter is really really hard to test for.

Science is a process, not a big book of answers. If you want a big book of answers there are any number of religions willing to accommodate you. Just be aware that the answers you get may be (1) vague, (2) contradictory and (3) of limited predictive use.

Re:Check the Phlogiston Compensators

[Anonymous Coward]

that was an awful lot of words to say "i should shut up"

oh, it's there,

[Thud457]

it's spiders.
teeny-weeny black spiders.
hundreds of Quattuordecillions of teeny-weeny black spiders per cubic centimeter, crawling between the very fabric of creation.
crawling in your ear, in your eye.
SPIDERS.

It is very hard to avoid dark matter

[amaurea]

The main lines of evidence for dark matter:

* Galactic rotation curves
* Velocity distribution in clusters of galaxies
* Gravitational lensing in general
* The Bullet Cluster in particular
* The pattern of positions of galaxies in the universe
* The pattern of Baryon-acoustic oscillations in the cosmic microwave background and in the galaxy distribution
* The primordial distribution of light elements in the universe

We know of some kinds of dark matter already: There is a huge amount of neutrinos left over from the big bang, and since these interact very weakly with other stuff, they definitely qualify as dark. Other known kinds of dark matter are black holes, and compact, cold objects made out of baryons (normal matter). So dark matter exists.

The problem is that there isn't enough of the normal kinds of dark matter. To match the pattern in the cosmic microwave background and the amount of hydrogen, helium and lithium in the universe, one needs by far most of the dark matter to be non-baryonic (i.e. not normal matter, but something like neutrinos, but heavier). This kind of dark matter is something we have to postulate exists in order to match observations. But when we do assume it exists, the theory matches observations extremely well. As an example, look at the CMB power spectrum as mesured by Planck. The error bars are so small that you mostly can't see them, and the points lie smack on top of the theory curve. But only if dark matter is included.

And it just so happens that the amount of dark matter that makes theory match the points in that graph also makes the element abundances, galaxy distribution, lensing observations and galaxy cluster velocities work too. Such a coincidence is pretty telling, I think.

But yes, people have tried to avoid dark matter by modifying gravity instead (though nowadays, the most common motivation for modifying graivty is to avoid dark energy). MOND is an example of that. MOND is like normal Newtonian gravity as long as the gravitational acceleration is large (like in the solar system), but instead of falling to arbitrarily low values as distances increase, the gravitational acceleration has an effective minimal value that it approaches as you move away. And such a constant value is just what you need to get the flat rotation curves we observe in galaxies. Which is the problem MOND was invented to solve.

MOND is an elegant solution for galaxies, but it loses all its elegance and predictive power when you try to apply it to the other areas where dark matter shows up. And in some cases it is plainly ruled out as an explanation. MOND, like Newtonian gravity, is a central force, which means that the force points towards the mass that generated it. But in the Bullet cluster, the gravitational force points towards areas with little visible matter, away from areas with much visible matter. This is impossible to fit into MOND. So the Bullet cluster basically killed MOND.

Some of MOND lives on in TeVES, which is an attempt at a relativistic version of MOND. Sadly TeVES has none of the simplicity and elegance of MOND, and while it can explai

Re:Have they considiered...

[amaurea]

Actually, it separated the hot gas in the galaxies from the stars and dark matter in the galaxies. Stars are so small compared to the distances between them that when galaxies collide, the stars just pass right through each other. The same applies to the dark matter (because it doesn't interact electromagnetically (or it would be visible), it does not experience any significant friction force). But the diffuse, hot gas collides and gets left behind in the collision. So you end up with dark matter and stars on each side of the collision point, and a huge amount of hot gas stuck in the middle. That gas is much heavier than the stars, so without dark matter, the gravitational field should be concentrated around the gas. But instead we see it (through gravitational lensing) to be concentrated around the stars (which is where we would expect the dark matter to be as explained above).