Dark Matter Exists

olclops writes "It's a big day for astrophysics. After much speculation, scientists now have conclusive proof of dark matter. This result doesn't rule out alternate gravity theories like MOND, but it does mean those theories will have to account for exotic forms of dark matter."

Dark Matters

The announcement [slashdot.org] of the pending announcement [nasa.gov] regarding Dark Matter [wikipedia.org]

"This is the most energetic cosmic event, besides the Big Bang, which we know about," said team member Maxim Markevitch of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

I guess he's never heard of Zaphod Beeblebrox.

"A universe that's dominated by dark stuff seems preposterous, so we wanted to test whether there were any basic flaws in our thinking," said Doug Clowe of the University of Arizona at Tucson, and leader of the study. "These results are direct proof that dark matter exists."

Also a bit of info on physorg [physorg.com]

How does the Coalsack Nebula [seds.org] fit into this? It's dark and it's matter, right?

Re:Dark Matters

Is it proof that dark matter exists that the link takes me to a blank page???

Re:Dark Matters

In case your question is not meant to be humorous, the Coalsack Nebula is not 'dark' in the same sense as dark matter. It's conventional matter that is not well lit.

Re:Dark Matters

I'm not an astrophysicist, so feel free to consider this only mildly informed:

What they claim to have found is a very hot galaxy undergoing gravitation not explainable by the conventionally visible matter.

All of the conventional matter in the area should be hot enough to be conventionally visible.

But since they can't see enough matter to account for the gravitation we have to conclude:

1) It's dark matter. That mysterious stuff that just doesn't interact like conventional matter, but does cause gravity.
2) It's conventional matter in some seriously surprising state that we don't understand, causing it not to be visible.

And their conclusion is that #1 is the more likely explanation. #2 seems unlikely because you would expect to observe this surprising state in the local galaxy or in experiments we perform in colliders.

Re:Dark Matters

"2) It's conventional matter in some seriously surprising state that we don't understand, causing it not to be visible"

What... like, being behind other matter? :-p

So what's new, then?

So what's new then? All along the whole case for "dark matter" was that galaxies -- _all_ galaxies -- rotate strangely like a rigid body, except right near the centre. According to newtonian mechanics the stars in a galaxy should behave basically like the planets in our solar system: the farther from the centre you get, the slower they move. But in a galaxy stuff moves like that only near the centre, and then it's like gravity changed gradually from 1/(R*R) to 1/R, and the stars rotate at an almost constant angular velocity around the centre.

So from there it's that either:

1. there's a metric buttload of matter we can't observe other than through gravity, in some weird distribution all through the galaxy's disc, or 2

2. we accept that gravity isn't working like we think it does

(Or my favourite: 3. galaxies are just a rotating texture there, so _of_ _course_ they rotate like a rigid. Noone would be dumb enough to simulate the individual stars just to give us a pretty sky in this MMO we call RL ;)

And somehow the favourite is 1, for no obvious reason than that noone wants to modify gravity theories. It's as if Galileo, upon discovering that a stone dropped from the mast doesn't lag behind the ship, would then proceed to invent some "dark wind" that pushes the stone along with the ship. Since existing wind obviously isn't strong enough to push the stone that hard, it's got to be some dark wind in there too. Just, you know, for the sake of not contradicting the existing Aristotelian system.

Anyway, all along we knew that it can't be conventional matter, because we already had plenty of galaxies in various states of illumination and they all behave the same.

So exactly how does the new one help there? It seems to me like it still can't offer conclusive proof that 1 is true and 2 is false, because it would _still_ be equally well explained by 2. What this "solves" is at most a sub-distinction inside 1, once we're dead-set on believing 1 instead of 2. It says basically that if we already decided it's 1, then, yep, it's definitely not baryon matter (rocks, gases, protons, etc), but some weird matter that interacts only with gravity.

Re:So what's new, then?

You make good points, but the devil is in the details. Long post follows :

Relative to your point (2) Some scientists have proposed that indeed gravity doesn't work as simply as General Relativity explains it, in particular, the simplest one called MOND assumes that gravity weakens with distance. It is sufficient to explain the rotating galaxy artifacts that you mention, however MOND is purely phenomenological, in other words it does not provide a mechanism by which gravity should weaken. It can be adjusted to rotating galaxies observations by modifying a couple of parameters, but it explains nothing.

With regards to (1), scientists are loath to abandon GRT because it is funded on very simple principles (essentially everything is local and the effects of acceleration and gravity cannot be distinguished) and explains so much with so little. There are myriads of ways to extend GRT in such a fashion as to explain observations by playing with the equations but AFAIK none can be derived from simple first principles unlike plain Jane GRT.

Indeed the simplest explanation to the observations is to admit that there is a great deal of matter in the universe that doesn't interact with normal matter as usual (it doesn't heat up in the same way for example) and is therefore dark, but does possess mass and affects observations. Of course it looks as if an enormous list of free parameters has just been added to GRT, but this is not innocuous. Dark matters, if it exists, should show up in observations other than with rotating galaxy data.

Now the new data is not derived from rotating stars but from large clouds of galaxies attracting each other. This is precisely why this is interesting, because it does look as if the new data confirms the existence of some kind of matter that doesn't heat up in the normal way and attracts normal matter, but this time not in a rotating framework, more in a translating framework. This is something that MOND does not explain.

Also perhaps we can design experiments that would prove the existence of dark matter in the lab. We already know about neutrinos, which fits the description of a kind of dark matter. Neutrinos do not interact through the electro-magnetic force or the strong force, they don't interact with normal matter, they don't heat up. They are very hard to observe due to this fact, and to characterize. However we have been able to prove their existence in the last few decades via indirect effects, and to prove they have mass. Neutrinos are very light though, we would need absolute humongous amounts of them to explain the vast quantities of dark matter that would explain the observation, and thus a mechanism that would generate such huge quantities of neutrinos.

Or perhaps there exists other kinds of weakly interactive particles that are much more massive. This is not predicted by the standard model of quantum mechanics though.

So right now physics is at an impasse : either GRT is wrong or QM is wrong. Probably both in fact, but what we do like is a smallish set of first principles that would guide us towards a better, more comprehensive theory. It was hoped that superstrings would be it, but it's too complicated and right now untestable.

The key points in conclusion : yes you can propose changes in the way gravity works to explain older observations, and some scientists have done so. However these changes are not popular because they are essentially ad hoc and explain nothing. Furthermore the latest observations seem to imply they are not sufficient anyway. Dark matter explains both old and new observations, but we don't know what dark matter is, how it is produced and how to characterize it. Right now this is not satisfactory, but this means new awesome discoveries are awaiting us in the (hopefully near) future. Stay tuned !

I hope this helps.

Re:Dark Matters

Indeed, there is no such thing as "proof" in science. Merely observations that support a current theory. I guess "we observed phenomena consistent with a theory that claims dark matter's existence" even less headline-worthy.

Re:Dark Matters

observed phenomena consistent with a theory that claims dark matter's existence

Or "evidence," for short.

Re:Dark Matters

You can prove things in math. Math isn't science. Math is a special construct where you make a few assumptions and then spend eternity figuring out what the consequences of those assumptions are. Thus, when you prove something in math you're really saying that X is always true assuming your axioms.

In science you don't get to make up the axioms. The universe does and we get to try to figure out what they are.

Re:Blog First, Then Scientific Journals.

Funny, I always figured the announcement of experimental confirmation of dark matter would first be published in a scientific journal or announced at a news conference...not on a blog shared by Mark, Claire, and Sean, whoever they may be.

I'm outraged -- are you really implying that we should take this proof of dark matter with a grain of salt, while there's this well-known Irish company that's using dark matter to produce free, clean and constant energy right now?

Re:Blog First, Then Scientific Journals.

http://www.sciencedaily.com/releases/2006/08/06082 1133930.htm [sciencedaily.com]
is this good enough for you?

Re:Blog First, Then Scientific Journals.

maybe we catch a peak

or we might catch a trough. Depends if you're a pessimist or an optimist.

Re:Blog First, Then Scientific Journals.

Funny, I always figured the announcement of experimental confirmation of dark matter would first be published in a scientific journal or announced at a news conference...not on a blog shared by Mark, Claire, and Sean, whoever they may be.

They're physicists (I think Sean Carroll works in cosmology, formerly of the U. of Chicago, now at Cal Tech). It was announced, and the paper has been written. The blog, by the way, is really good.

Re:Blog First, Then Scientific Journals.

I can't speak for the others, but Mark Trodden and Sean Carroll are theoretical cosmologists. I majored in astrophysics as an undergrad and read some of their papers. Also, Sean Carroll is quoted in the press release [harvard.edu] on NASA's web site.

So these aren't just random guys talking, but professionals in the field. Also, as Sean states in his post, the result was embargoed, which means it was being kept under wraps before publication in a journal. This article and the one I mentioned above are just talking about the results that are published elsewhere. If you really want to read the journal article, it's available here [harvard.edu].

Re:Blog First, Then Scientific Journals.

The key quote from the paper is:

Any non-standard gravitational force that scales with baryonic mass will fail to reproduce these observations.

This is a very nice piece of work. One observation doth not a proof make (the myth of the "crucial experiment" is, well, a myth) but if confirmed by comparable observations on similar structures it could really start to constrain inter-galactic dark matter models in ways that are much more precise than hitherto has been possible.

The fundamental importance of this paper is less in the single observation than in the development of a new technique for probing the inter-galactic dark matter distribution directly and in detail.

Of course, it says nothing at all about galactic dark matter.

Re:Blog First, Then Scientific Journals.

It didn't "Melt Down".

It underwent a resonance cascade reducing the server into its Dark-Matter counterpart.

Sheesh, get with the program here!

Re:Dark Matters

How's this: imagine that there is some being which created everything by magic. Did he create himself? Was he himself created? Sure it's also preposterous, but consider it as an alternative.

Re:Dark Matters

OMG its a wachowski brother!!!

Re:Silly Musings.....

There are very few free protons or free electrons, and no free neutrons (half-life of about 15 minutes before it turns into hydrogen) - nearly all interstellar matter is composed of hydrogen and helium. Beyond which, by your theory they would be generating an absolutely massive electromagnetic charge.

Beyond that, though, it's estimated that about half of baryonic matter is invisible for various reasons - thus, the Universe appears to be composed of 2% luminous baryonic matter, 2% invisible baryonic matter, 23% dark matter and 73% (and increasing) dark energy.

Re:Silly Musings.....

Couple of things ---

Zeroth, stray neutrons decay to a proton, electron, and an electron anti-neutrino. n->p+n doesn't conserve charge.

First, "ordinary" baryonic matter like protons can only be (according to the well-verified Big Bang Nucleosynthesis) a few percent of the total mass density of the universe, and perhaps 10% of the total amount of Dark Matter. We think the dark stuff is largely, if not almost completely, non-baryonic (not made of quarks, not strongly interacting).

Next, for any isolated mass of protons (essentially ionized H), you'd have to explain where all the electrons went, since the Universe appears to be electrically neutral on even small scales. Also, since the electric force is so overwhelmingly much stronger than gravity, any such cloud cannot be gravitationally bound and would explosively disperse. It wouldn't be perfectly transparent, since protons (being charged) have some cross-section to scatter photons just like free electrons do. In fact, the X-ray emission mentioned in the article comes from hot, ionized H.

I'd like to post a mirror but...

...the server must have known of the impending slashdot effect and preemptively protected it's CPU from the impending meltdown

Full Paper

The full paper can be found here. From the abstract: [harvard.edu]
We present new weak lensing observations of 1E0657558 (z = 0:296), a unique cluster merger, that enable a direct detection of dark matter, independent of assumptions regarding the nature of the gravitational force law. Due to the collision of two clusters, the dissipationless stellar component and the fluid-like X-ray emitting plasma are spatially segregated. By using both wide-field ground based images and HST/ACS images of the cluster cores, we create gravitational lensing maps which show that the gravitational potential does not trace the plasma distribution, the dominant baryonic mass component, but rather approximately traces the distribution of galaxies. An 8 sigma significance spatial offset of the center of the total mass from the center of the baryonic mass peaks cannot be explained with an alteration of the gravitational force law, and thus proves that the majority of the matter in the system is unseen.

Re:Full Paper

Now hold on a second, I'm not an astrophysist, I'm a chemist, but let me apply a little scientific reasoning to your last sentance An 8 sigma significance spatial offset of the center of the total mass from the center of the baryonic mass peaks cannot be explained with an alteration of the gravitational force law, and thus proves that the majority of the matter in the system is unseen

How exactlty does demonstrating that something cannot explain a phenomina prove that a counter argument is proven? That's like saying the spontaniuos combustion of my dog cannot be proven with an alteration of the gravitational force law, and thus proves that the majority of the matter in the system is unseen.

Re:Full Paper

I know your post was somewhat tongue-in-cheek, but it was modded interesting, so others are probably wondering the same thing. Lets play the analogy game first...

We know musical talent must come from either training or predisposition. We assume there are no other factors, because those two cover the reasonable possibilities. Consider this logical statement then: John Doe has no musical training, yet he is very skilled. John's musical skill cannot be explained by training, and thus proves that there exists some sort of predisposition to musical talent. It doesn't tell us (e.g.) whether it's genetic or not, but knowing for sure that it's there helps us refine our further studies.

Now the real version. There is "more" gravity than we can account for with the combination of Baryonic (regular) matter and Einstein's theories of gravity. A LOT more. There are only two possibilities: Gravity gets stronger under certain conditions (regular matter pulls harder), or something "unseen" is pulling. Of course, both could be true, but at least one of them MUST be true to match observations. We assume there are no other explanations, because those two are broad enough to cover the entire range of reasonable possibilities.

This experiment showed that the center of gravity of certain galaxies doesn't correspond to the center of the regular matter. In other words, the galaxy's gravity is pulling in a different direction than the normal matter would indicate. "[This] cannot be explained with an alteration of the gravitational force law, and thus proves that the majority of the matter in the system is unseen."

More info from a server that's not on fire...

Here's some info from NASA. [nasa.gov]

Somehow I also think...

It's a big day for astrophysics.

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I think it's going to be a big day for their webmasters as well.

Sweet!

Now it's our turn to hide from the dark matter and wait for it to discover us! Come one everyone - pick a hiding spot and get to it! Hurry!

and, presumably...

does this mean grey matter exists as well?

Better Links?

For those who prefer here are the salient links which TF"A" (it's a blog entry) is referencing: http://chandra.harvard.edu/chronicle/0306/devil/ [harvard.edu] http://chandra.harvard.edu/press/06_releases/press _082106.html [harvard.edu]

i don't believe it

After much speculation, scientists now have conclusive proof of dark matter.

Well, their proof is based on the detection of gravity and gravitational fields. Every real American knows that it's not "gravity", but "intelligent falling". Gravity is a myth invented by foreign scientists to make all Americans seem overweight.

The new result, in a nutshell

Astronomers observed a distant cluster of galaxies in optical light, with ordinary telescopes, and in X-ray light, with a telescope in space. This is an unusual cluster of galaxies, since there is clear evidence that one small group of galaxies are "interlopers:" members of a smaller cluster which fell into a larger one some time ago. Members of this interloping group are all bunched together at one side of the main cluster.

The visible light image shows the galaxies within the cluster. It also shows, much fainter and much smaller, a very large number of BACKGROUND galaxies -- these are objects way, way farther away than the big cluster. As the light from these background galaxies passes through the big cluster, it is bent very slightly by the gravitational field of the cluster. This gravitational lensing distorts the shapes of the faint, little background galaxies just a bit, but with care, we can measure the effect. We learn from the lensing where the matter is in the cluster: that is, we can figure out where the stuff which produces gravitational effects is distributed. That's part one: a map of the matter within the cluster, based on gravitional lensing.

The X-ray image shows emission from hot gas within the cluster. We have known for several decades now that large clusters of galaxies are immersed in giant clouds of very hot gas, at temperatures of millions of degrees. The gas emits copious amounts of X-rays. In most clusters, the amount of this hot gas -- its total mass -- is much larger than the amount of mass we can see in stars. That is, counting the stars in the galaxies suggests a total amount of mass-in-stars M, but computing the amount of hot gas necessary to emit all the observed X-rays yields a mass-in-hot-gas of around 10*M, ten times as much.

On the other hand, the amount of mass derived from the gravitational lensing of background galaxies is about 10 times larger still, or about 100*M. The stuff which produces the gravitational lensing does not emit visible light, nor X-ray light, nor, as far as we can tell, any electromagnetic radiation. Therefore, we call it "dark matter". It produces a gravitational force, but that's about all we know about it. (There are additional reasons for believing that this mysterious stuff is not made up of electrons, protons and neutrons, but that's another story).

This new result is interesting for this reason: the X-rays appear on one region of the cluster of galaxies, telling us that the bulk of the ordinary matter is RIGHT HERE. The map of total mass we can make from gravitational lensing appears in a different region of the cluster, telling us that the bulk of the dark matter is OVER THERE. It is very clear that the dark matter and ordinary matter are distributed in different places. This isn't too surprising, perhaps, if one small group of galaxies rammed into a big cluster -- the gas ram pressure might push on the ordinary hot gas in a different way than on the dark matter (which wouldn't feel any ram pressure at all, actually).

As Martin Hardcastle pointed out to me in a Google newsgroup a few days ago (thanks, Martin!), this is certainly not the first evidence for dark matter -- we have a number of examples in which gravitational forces are larger than the amount of visible matter would suggest -- but it is the first good case in which the distribution of the dark and ordinary matters are so clearly displaced.

credible and accurate

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I haven't yet read this article due to it being slashdotted, but I'm sure it is at least as credible as the story about the new source of free energy from magnets and as accurate as the one that says goldfish are smarter than dolphins.

Kanye West says..

George Bush hates dark matter

MOND

From the title - "This result doesn't rule out alternate gravity theories like MOND". Actually, this directly rules out MOND. That's a big part of the point of the experiment.The idea is that the mass in these clusters doesn't come from the obvious sources of visible matter (the gas), as it would in a MOND or normal gravity scenario, but rather from the invisible (i.e., dark) matter.

"To be published..."

From the NASA press release: "These results are being published in an upcoming issue of The Astrophysical Journal Letters."

Two points. First, journals really hate it when press releases are made prior to the publication date. Second, this journal has an "impact factor" of ~5-6, compared to Nature, or Science, which have impact factors of ~25. Why are they publishing in some obscure journal if this is really the rock-solid proof that they claim it is?? Makes me wonder.

Re:"To be published..."

ApJ Lett. is as good as it comes in astronomy. ApJ is the most significant journal in astronomy, followed by Astronomy and Astrophysics. Partly this is on volume (ApJ is huge) - I don't know how the impact factors compare. ApJ Lett. presumably has higher impact factor than ApJ as a whole.

Bad Nibbler !

A steaming pile of Dark Matter was found on the sidewalk and traced back to Nibbler's [wikipedia.org] litterbox. Bad Nibbler!

This is What Slashdot Should Be

Did anyone else notice the amazing quality of TFA? I actually understand more about dark matter from that article than from anything else I have read on the subject to date. This makes me less grumpy about all the money I felt was "wasted" on telescopes vs. planetary exploration.

As I understand the process ...

... this would constitute a confirmation if a prior prediction was made, and the observed results match the prediction.

This appears to be no more a confirmation for dark matter than when the Michelson-Morley experiment (in 1881) "confirmed" the existence of ether. In the immediate aftermath of the Michelson-Morley experiment, theoreticians generated lots of mathematical "proofs" (e.g., The Ether of Space, Sir Oliver Lodge, Harper & Bros, 1909 [google.com]) that showed how a boundary layer in the ether surrounding the Earth accounted for the observed results. A series of subsequent refinements of the Michelson-Morley experiment showed that the speed of light was truly independent of direction, and Einstein's theories, which did not require the existence of ether, provided a better fit for the observed results than was a boundary layer in the ether.

Over time, the Michelson-Morley experiment was recognized to have disproved the existence of ether -- but it wasn't that way initially.

Alternative explanations include "quantum critical phase transitions [newscientist.com]", and I'm sure that there are other possibilities, that a series of observations of similar cosmological events will provide the range of data needed to select the hypothesis that best describes the observations.

Being able to fudge one theory to fit a single observation falls quite a bit short of a "conclusive proof". Maybe dark matter does exist, but it's going to take a lot more observations for it to be convincing to me.

How precisely does dark matter permit the expansion of the universe to be defined, and how precisely does the observed phenomenon fit those numbers?

Wake me up when someone has a quantum mechanical model that tells how quarks are bound together in dark matter, or when someone manages to tap into dark energy (which is supposedly all around us).

Full-text from Browser Cache...

Dark Matter Exists [cosmicvariance.com]
Sean at 11:52 am, August 21st, 2006

The great accomplishment of late-twentieth-century cosmology was putting together a complete inventory of the universe [uchicago.edu]. We can tell a story that fits all the known data, in which ordinary matter (every particle ever detected in any experiment) constitutes only about 5% of the energy of the universe, with 25% being dark matter and 70% being dark energy. The challenge for early-twentyfirst-century cosmology will actually be to understand the nature of these mysterious dark components. A beautiful new result [harvard.edu] illuminating (if you will) the dark matter in galaxy cluster 1E 0657-56 is an important step in this direction. (Heres the press release [harvard.edu], and an article in the Chandra Chronicles [harvard.edu].)

A prerequisite to understanding the dark sector is to make sure we are on the right track. Can we be sure that we havent been fooled into believing in dark matter and dark energy? After all, we only infer their existence from detecting their gravitational fields; stronger-than-expected gravity in galaxies and clusters leads us to posit dark matter, while the acceleration of the universe (and the overall geometry of space) leads us to posit dark energy. Could it perhaps be that gravity is modified on the enormous distance scales characteristic of these phenomena? Einsteins general theory of relativity does a great job of accounting for the behavior of gravity in the Solar System and astrophysical systems like the binary pulsar, but might it be breaking down over larger distances?

A departure from general relativity on very large scales isnt what one would expect on general principles. In most physical theories that we know and love, modifications are expected to arise on small scales (higher energies), while larger scales should behave themselves. But, we have to keep an open mind in principle, its absolutely possible that gravity could be modified [blogspot.com], and its worth taking seriously.

Furthermore, it would be really cool. Personally, I would prefer to explain cosmological dynamics using modified gravity instead of dark matter and dark energy, just because it would tell us something qualitatively different about how physics works. (And Vera Rubin agrees [newscientistspace.com].) We would all love to out-Einstein Einstein by coming up with a better theory of gravity. But our job isnt to express preferences, its to suggest hypotheses and then go out and test them.

The problem is, how do you test an idea as vague as modifying general relativity? You can imagine testing specific proposals for how gravity should be modified, like Milgroms MOND [wikipedia.org], but in more general terms we might worry that any observations could be explained by some modification of gravity.

But its not quite so bad there are reasonable features that any respectable modification of general relativity ought to have. Specifically, we expect that the gravitational force should point in the direction of its source, not off at some bizarrely skewed angle. So if we imagine doing away with dark matter, we can safely predict that gravity always be pointing in the direction of the ordinary matter. Thats interesting but not immediately helpful, since its natural to expect that the ordinary matter and dark matter cluster in the same locations; even if there is dark matter, its no surprise to find the gravitational field pointing toward the visible matter as well.

What we really want is to ta

Re:Dark matter and tech

I say we use it to build a dyson sphere around the entire universe. Then we can finally solve the question of whether the universe is expanding / contracting / balancing. The hard way.

Re:So funny

We know nothing, and yet you can post bitchy comments to slashdot, on a computer, connected to the Internet, powered by a physical plant hundreds or thousands of miles away piping electricity directly into your laptop, and then watch a show on TV over cable distributed by satellite.

I'd hazard a guess that we actually do know a thing or two.

Re:So funny

"we know nothing"

A mistake known as "generalising from self"; you know nothing, therefore you assume that everybody else must know nothing too (not your fault, you cannot conceive of anything else). This is a primitive form of reasoning called "induction", whilst it can have its place, it often leads to huge inaccuracies such as deriving "we know nothing" from "I know nothing".

Proof of your limited ability to use logic:
"we know nothing, and what we do know..."
The two are mutually exclusive; we cannot both know nothing, and have stuff that we do know.

Proof of your lack of knowledge:
"Science-is-infallible types claim to know and understand the universe"
No they don't. They claim to be trying.

With both a lack of knowledge and a lack of ability to use logic, one might think the two would cancel out and you'd get a thing or two right, but I guess you're pretty unlucky, which would explain the bitterness too.

Re:I don't see any proof...

I didn't understand it until I watched the video.

Essentially it goes like this. They see a collision and make an assumption about what it was that collided.

Then, they looked over the area and determined where the mass is right now (from our point of view).

If the assumption about what collided is correct then the result should have been a mass of hot gas that is distributed like you'd expect if a ball of hot gas collided with another ball of hot gas.

Dark matter supposedly only interacts by gravity. Normal matter interacts by gravity plus nuclear and electromagnetic forces. That means that in a collision, normal matter collides with other normal matter while dark matter is merely slowed down and pulled by gravity.

The mass distribution that they observed matched up with the mass distribution implied by the dark matter theory. It can't be accounted for with just normal matter.

The parts of the theory that would need to hold up:
- the assumed initial configuration of the matter before collision.
- the current mass distribution that they observed.
- the calculation about how the collision should behave if it's all normal matter.
- the calculation about how the collision should behave if it's part dark matter.

If those parts hold up then it's a pretty striking discovery.

Remember Vulcan? ( no, not startrek vulcan )

Remember Vulcan, the planet with an orbit inside of Mercury? It was PROVEN to exist in the late 1800s. The calculations showed that Mercury's orbit required a smaller planet to make Mercury's orbit precess as it did. People even went looking for it with the finest telescopes of the day. And they saw it.
Then some smart aleck who worked in a patent office came along and showed that space is warped and that Mercury's orbit fits perfectly. Vulcan disappeared, never to be seen again.

Vulcan had more data in favor of its existence back then than dark matter does now. Pardon me, but I'm as skeptical as parent.