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There's more gravitational force in our galaxy (and others) than can be explained by counting stars. So why not lots of dark planetery systems (ie without stars) ? Why must we assume some undiscovered and unexplained form of matter ?

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In the Solar System, the star is 99.8% of mass of the whole. –  mbq Nov 17 '10 at 20:55
    
@JohnFx and McGarnagle: At one time neutrinos would have seemed like a last ditch attempt to prop up an old theoretical idea (conservation of energy/momentum) that many at the time were actually considering giving up, despite a complete lack of positive evidence for their existence. Then they were found experimentally. Just because something looks vaguely like a previous episode from the history of science doesn't mean it will play out the same way. We are in new territory that hasn't been seen before. Only new experiments will settle the question once and for all. –  Michael Brown Nov 1 '13 at 2:58
    
We also have the successful discovery of the outer planets when orbit predictions were wrong, contrasted with the false prediction of Vulcan that was remedied by GR. Both methods (new object vs new theory) have worked, and both have failed. Saying it's reminiscent of some previous attempt or other is a completely empty statement. –  Robert Mastragostino Nov 1 '13 at 3:13

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There is a very precise reason why dark planets made of 'ordinary matter' (baryons - particles made up of 3 quarks) cannot be the dark matter. It turns out that the amount of baryons can be measured in two different ways in cosmology: - by measuring present-day abundances of some light elements (esp deuterium) which are very sensitive to the baryon amount, and - by measuring the distribution of the hot and cold spots in the Cosmic Microwave background (CMB), radiation left over from the early universe that we observe today

These two methods agree spectacularly, and both indicate that baryons are 5% of the total stuff (energy/matter) in the universe. Meanwhile, various measures of gravitational clustering (gravitational lensing, rotation of stars around galaxies, etc etc) all indicate that total matter comprises 25% of the total. (The remaining 75% is in the infamous dark energy which is irrelevant for this particular question).

Since 5% is much less than 25%, and since the errors on both of these measurements are rather small, we infer that most of the matter, about 4/5 ths (that is, 20% out of 25%) is 'dark' and NOT made up of baryons.

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As above answers have mentioned most of the ordinary matter has been considered as candidates and we are fairly certain that there has to be some sort of "dark" matter at work.

Firstly, we take on the phenomenon of gravitational lensing. A very famous example is the Bullet cluster where you can clearly observe the effects of a compact mass acting as an optical lens. A mass of such magnitude can not conveniently be a cluster of brown dwarves and most black holes we observe have many objects orbiting and a particle jet accompanying. Besides, a black hole of small magnitude would not be enough to explain the magnitude of the gravitational lensing in question.

Secondly, much more complicated computations made with general relativity principles need a lot more mass to account for the current shape and angular speed of galaxies. It may be easy to say that GR is wrong(which you shouldn't say to the face of a physicist), even though GR is incomplete in the sense that it is not a theory of everything, it still explains most of the gravitational phenomena pretty well. Besides, the fact that there is gravitational lensing means that there really IS some sort of mass or gravitational pull(or more like space time curvature anomaly) in certain parts of our galaxy, and the universe.

Even though dark matter seems so mysterious, we can guess most of its properties from our "lack of knowledge". Firstly, since it is "invisible" in all spectrum of light, we can assume that it does not interact via electromagnetic force. Since it is a fountain of gravitational force, we can say that it, surprise surprise, interacts via gravitational force. Calculations for weak&strong forces are pretty complicated and very indirect so I'm just going to say that most of the currently proposed dark matters interact via strong and weak forces as well.

A planetary system without a star is only possible if the star died out to be a neutron star, black hole, or a white dwarf, all of which are detectable in some way.

Besides, there would have to be an unrealistic amount of these "faded" star systems to even account for the lost mass. And assuming that such planetary systems are fairly rare, we would rather consider an unusual form of matter which is quite possible and plausible if found.

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The major missing link is the existence of a class of particles, which aside from gravity are sufficiently weakly interacting to fill the need. We already know of one kind of particle which has most of these characteristics, neutrinos. In this case physicists and cosmologists have theoretical reasons to believe the upper bound on the amount of mass in neutrinos is too small. But the mere existence of one class of weakly interacting particles should make the possibilty of another appear less novel.

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All the matter that we do know to exist (called Baryonic matter) emits some kind of electromagnetic radiation at some frequency. Sometimes it's measured in infrared radiation, because matter, no matter how cold, will still radiate some amount of heat. To the best of our knowledge, it's not actually possible to cool any matter to absolute zero, and it's certainly not happening naturally. I believe the coolest matter known to exist in the universe is around 3 degrees Kelvin. Other wavelengths can determine the exact nature of the matter in question, and its temperature. For example, ionized oxygen glows in visible light at a certain wavelength - that's one of the things that make emission nebulae visible.

So, after measuring all that matter in other galaxies, we've noticed that there's simply not enough mass to keep them from falling apart (namely, stars within galaxies are moving faster than the escape velocity of the gravitational pull of the center of mass of the galaxy) - especially when we've tried to make computer models of galaxies. That was the first clue that there was something going on that we didn't fully understand. As others have described, other methods of determining the total mass of a galaxy have shown similar results.

As a result, there's only one possible explanation for this phenomenon: there must be some kind of matter (and all matter has mass) that we can't detect. In fact, the amount of mass that can't be accounted for in this way is about 95% of the mass of any given galaxy. Of course, that's a pretty big deal.

By the way, we've been trying to work out this very large discrepancy since 1933. The difference between now and then is that accuracy of our measurements of velocity of stars in other galaxies has improved, as well as our ability to measure other phenomena happening within galaxies and galaxy clusters. The more accurate we've become in our measurements, the more this glaring discrepancy has become evident.

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stars within galaxies are moving faster than the escape velocity of the gravitational pull of the center of mass of the galaxy Thanks for this concise description. Is this line of reasoning affected at all by general relativity, given that "escape velocity" is a classical concept? –  McGarnagle Oct 20 '12 at 19:39
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If you graph the velocity vs distance from the galactic center, you get a wildly different graph than you'd get from the expected outcome, even using Einstein's theory of gravity. It also demonstrates that the mass of the galaxy is very diffuse. Without the extra mass, stars at the extremities of the galaxy simply would not be a part of the galaxy at all. That's escape velocity. Here's an example graph: physics.uoregon.edu/~soper/Mass/galaxymass.html –  Ernie Dec 11 '12 at 20:53

The possibility of large dark objects made of normal baryonic matter has been considered. These are called MACHOs.

However, there are various reasons to think that most of the dark matter can't be in the form of MACHOs. From the above wikipedia article (which links to some relevant journal articles):

The Big Bang as it is currently understood simply couldn't produce enough baryons without causing major problems in the observed elemental abundances,[6] including the abundance of deuterium.[7] Furthermore, separate observations of baryon acoustic oscillations, both in the cosmic microwave background and large-scale structure of galaxies, set limits on the total baryon-to-total matter ratio. These observations show that a large fraction of non-baryonic matter is necessary regardless of the presence or absence of MACHOs.
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The micro-lensing studies during the '90s demonstrated that such objects compose a non-trivial, but sub-dominate fraction (not my field and I've forgotten just how much) of the "missing" mass needed to explain galactic rotation curves. But--as you say--that leaves it as a very small fraction of the mass needed to explain the large scale structure of the universe. –  dmckee Nov 17 '10 at 18:54
    
The Wikipedia page involves circular logic. Our model says that you can't have more baryons than what our model predicts, so Dark Matter can't be made up of baryons. If the model was so good, it would tell you where all the Lithium 7 isotopes are. –  DRAirey1 Nov 1 '13 at 0:25

All possible dim/dwarf/died stars, interstellar medium(gas,dust,molecular cloud etc.), have been considered. But they are not enough. The rest part can only be something we never know before. We are expecting some unknown new particles.

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