(Missing) dark matter isn’t a theory, per se; it is a placeholder for mass that appears to be out there but which we cannot currently observe. There are a number of hypotheses for what dark matter could be comprised of, and some of those hypotheses are at least technically falsifiable albeit not with current means, but none rises to the level of being an accepted theory. (A general rule of thumb in the physical sciences is that if an idea is published only in journals, it is hypothesis; if it is found in textbooks it rises to the level of accepted theory, and if it has someone’s name attached to it is probably considered a physical law or basic principle.)
The reason that most cosmologists and astrophysicists believe that dark matter is actually some kind of matter and not an artifact of some other modification to general relativity is because most of the tenants of general relativity have been tested to the highest precision we are capable and have held up completely. In fact, the only physical theory that has been more thoroughly tested to a higher degree of precision is probably quantum electrodynamics. The kind of modification to GR that would still align with current measurements and yet explain the large scale phenomenon we see would be very kludgy and inelegant (at least, for any proposals so advanced), and therefore, are probably neither accurate nor more useful than GR in its current form. So, dark matter remains, but everybody knows that it is a placeholder for some as-yet not completely specified influence.
The theory that’s being chucked is the one that says there’s nothing new going on. The dark matter saga is a beautiful example of scientific endeavors. No one’s saying, “Stuff doesn’t make sense, so invoke magic here… dark matter!” Rather, stuff doesn’t make sense, but it could make sense if certain types of matter were to exist, so maybe it does, and let’s go about trying to see if we can detect it or otherwise demonstrate that it doesn’t actually fix all observations by making appropriate measurements.
There are many independent indications of dark matter, and they all point to a consistent picture. Among them:
Galaxy rotation curves, i.e. how quickly objects rotate about the core of galxies, can’t be explained by standard gravity given the amount of visible mass. The right amount of additional mass would fix this.
Galaxy cluster behavior is inconsistent with the visible sources of gravitational potential energy present in the clusters, and this inconsistency can be fixed by assuming additional sources of gravitation.
The exquisite Bullet Cluster collision observations (and the gravitational lensing induced by the cluster) reveal distinct types of material present in the clusters, as stars, gases, and dark matter would evolve different through the collision process. Of note, these observations are incompatible with modified gravity models that could fix the rotation curves.
The cosmic microwave background radiation provides information on the matter and energy present very early in the universe, and our models to describe the CMB work amazingly well, from dead-on predicting the anisotropy spectrum in that link to other observables like how much hydrogen, helium, and lithium there should be floating around. It only works, though, if there’s some non-baryonic, heavy matter present.
Measurements of the details of large scale structure match the predictions of our cosmological model very well, but again only if some non-relativistic, weakly (or non-)interacting matter component is present during structure formation.
The cosmological models that yield the successful predictions of all this data can also be tested through other, independent, observations (supernovae, Lyman-alpha forest) that constrain different aspects of our predictions, and they all fit nicely.
Not only do these give a consistent picture for the presence and amount of dark matter (to within expected uncertainties), but you’d have to bend over backwards to explain away some of them without the relatively simple invocation of dark matter.
As for detecting dark matter: depending on its characteristics, you expect to see it directly in experiments by producing dark matter particles in collisions, by detecting the rare interactions of dark matter with physical detectors, or by detecting remnants of dark-matter-on-dark-matter annihilations in a space-based experiment. These aren’t the only ways at getting at dark matter, but they are the ones with most of the research effort at the moment.
Many people are excited also by the fact that the sort of particle that could make a good dark matter candidate would also fix a lot of known issues in the Standard Model of particle physics. Killing this second bird with an already needed stone would be a nice bonus.
In short, dark matter is far from an untestable pseudoscientific idea. On the contrary, it’s a terrific example of scientific progress over the past few decades. If you read over-simplified pop-sci headlines or articles, you could easily get a wrong impression. The real story is: this is solid, frontier science, and… give folks a chance already – this stuff takes time!
I was going to start a similar thread. I just have this strong suspicion that dark matter is the new luminiferous aether, that in a hundred years people will be smirking about.
So if dark matter makes up like 95% of the universe, how much of our solar system does it make up? Is it just stuff that’s everywhere else (but not here)? If so, why?
Dark matter is the mass of the gaps. First suggested in the 1930s.
But what evidence do we have that it exists? Well, we recently had gravitational lensing from a super cluster of galaxies that would seem to confirm that galaxies have as much dark matter as was posited to explain their rotation rates. Death to MOND! We now have new results coming out from a new satellite, this week in fact, that might shed some serious light on what dark matter is/might be.
We infer it is there from its gravitational effects on other matter. But we can’t see it.
That’s a good point. People always complain that dark matter seems an ad hoc addition in order to ‘fix’ the current paradigm, like epicycles were, but the additions you need to make in order to produce a new theory still consistent with observation are much greater kludges. For instance, the relativistic version of the leading dark matter alternative, MOND, is called Tensor-Vector-Scalar gravity (TeVeS). While dark matter models make the relatively simple addition of one or more new particles, TeVeS actually needs more than that: a scalar field and a vector field, plus several arbitrary functions and parameters. Plus, it is unclear whether it actually does the job we would want it to: in order to explain certain observations, we still need to require the existence of relatively massive neutrinos, IOW, a form of dark matter. (Some model calculations have also shown stars to be unstable on a scale of two weeks in this theory.)
Now there’s another candidate, Scalar-Tensor-Vector gravity (STVG) or more simply MOdified Gravity (MOG), which seems to fare better observationally, but which still needs to introduce an additional (‘fifth force’) scalar field, and actually elevate all the constants of the theory to the status of dynamical fields themselves. As before, this baroqueness doesn’t immediately rule it out, of course—that can only be done by experiment—but it doesn’t seem an obviously simpler account for observation than the dark matter hypothesis.
Additionally, several proposed extensions of the Standard Model of particle physics naturally give rise to additional particles that may fulfill the role of dark matter. One possibility is supersymmetry, in which there exists a ‘lightest supersymmetric particle’ that is absolutely stable (i.e. it does not decay) and generally only weakly interacting. Other candidates with independent motivation would be axions or massive sterile neutrinos.
I think you fundamentally misunderstand science. All that is needed for a hypothesis to be accepted is that it is more likely than the alternatives. The greater the discrepancy in likelihood, the more sure we are about the hypothesis. Those things we call theories just have a high confidence rate.
If science had to wait for some definitive evidence, we’d never learn anything. Right now, dark matter is the most likely explanation. Could we find new evidence that changes that likelihood? Sure. But we’re not going to just throw up our hands and say we don’t know at all, impeding progress in other areas.
I’m a bit confused as to why everybody seems to be making such a big deal out of this. Qualitatively, this tells us basically nothing new; we knew the positron excess was there from the results of PAMELA and Fermi, the new results just show it extends to higher energies (which might actually be a bad sign: if it is due to some dark matter annihilation, the excess should be expected to drop off). Also, everybody is pretty convinced that the PAMELA and Fermi results do not point into the direction of dark matter, at least not to any form of it that’s currently being discussed, and the new AMS results don’t really add anything in this respect.
True, but it is sometimes reasonable to postulate the existence of a currently unobserved phenomenon which helps to explain other, observable phenomena. I liken it to Mendeleev formulating the periodic table. He saw patterns that suggested the existence (and properties) of currently unknown elements and temporarily assumed their presence because it made other observations fit.
I don’t see a problem with provisionally assuming the existence of dark matter. If, later, a better explanation arises that postulate can be discarded.
Which misses the entire point of the scientific process. Who, to put it bluntly, fucking cares that the idea may lead to smirking in a hundred years? We’ll test it, find out and discard if needed. When you think about it, it’s almost a classic black box experiment that will be investigated directly and indirectly.
We can’t be certain about anything. I can’t be CERTAIN that there’s really a computer in front of me that I’m typing on. However, the assumption that there’s really a computer in front of me is the simplest explanation that fits the observed data.
And an important thing about the Luminiferous Ether, that also applies to dark matter: The scientific community did not say, “Well, then ether must exist – status quo saved”, and then went back to their offices to do whatever people did to dick around before Minesweeper was invented. They said “this really works out well if ether exists, lets see what we can find out about ether”. Experiments were performed determine the properties of the ether, and those experiments led to two conclusions: (1) ether doesn’t exist, and (2) relativity.
Science forces ideas to either be right or productively wrong. Dark matter either exists or trying to prove its existence will reveal new and interesting things.
Right. It’s not like scientist say, “Oh, dark matter. That explains it. Nothing more to see here, science is now over.” When you say dark matter, then you have a whole bunch of questions to ask, starting with, “What the fuck is dark matter?” Is it bigger than a breadbox? Can you build a bridge out of it? Can you detect it in any way? We know it interacts gravitationally, so it must interact with normal matter in some way, so it’s not just something that might as well not exist.
If, as time goes on, we show the kinds of qualities that dark matter must have if it exists, and then we look for things that have those qualities. And note that dark matter doesn’t have to be all one thing, it could be a mix of different things, although apparently only a very small fraction of it could be normal matter that’s just not emitting light, like interstellar planets.
Dark matter doesn’t make up that much of the universe. Dark energy makes up most of it. It is an extreme injustice to the science-interested public that the field has settled on the names “dark matter” and “dark energy” for these two very different things. Given the oft-discussed concept that matter and energy are related, it is reasonable for a casual observer to see an article about dark energy and conflate it with dark matter, or vice versa.
Dark matter seems to account for 27% of the total stuff in the universe. Ordinary matter: about 5%. Dark energy (unrelated to dark matter; also very different pieces of evidence, and with much less understanding) : 68%.
Short answer: unknown, but there would be very little of it compared to regular matter.
Most reliable quantitative information about dark matter comes from looking at phenomena that occur on length scales much larger than the solar system, so dark matter’s behavior on such puny length scales is still an open question. The distribution of dark matter in the galaxy might be smooth or clumpy, and the solar system might be in a clump or a non-clump. The galactic distribution might be spherical or might be flattened, depending on the dark matter’s self-interaction properties. There could be a large storehouse of dark matter that has collected in the sun. (Many folks are looking for evidence of this by searching for possible dark matter annihilation products emanating from the sun’s direction.)
All these niggling details notwithstanding, the general picture points to a tiny amount of dark matter in the solar system in comparison to regular matter. To be sure: this isn’t due to “too little” dark matter but rather due to the solar system’s having 10[sup]10[/sup] to 10[sup]15[/sup] times more regular matter than “average” space. (That’s what makes it a solar system, after all. It’s a clump of regular matter.)
This is why, for instance, you can’t look for deviations in the motion of the planets or whatnot. The expected effect is just too small. (Well, you can look, and it’s important to, but so far these measurements have led only to upper limits on the dark matter density, as expected.)
