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How do we know that dark matter is dark, in the sense that it doesn't give out any light or absorb any? It is impossible for humans to be watching every single wavelength. For example, what about wavelengths that are too big to detect on Earth?

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We have instruments actively probing the whole spectrum from the 2.7 K CMB up to the gamma spectrum. Really. Astronomers hate bands they don't have data in. – dmckee Feb 6 '11 at 1:42

If dark matter emitted very long wave lengths of electromagnetic radiation it would mean it is composed of charged particles. There is no escape from that conclusion. Somebody might propose that dark matter is some strange configuration of charged particles which acts as a very long wavelength antenna. That might be a good model, but there is a hitch with it. If it emits longer wavelength radiation it must be colder. The Wien displacement law is that $\lambda~=~c/T$, for $c$ a constant, which may be calculated, but is not relevant here. For $T~=~2.7$K the wavelength of radiation is $.1cm$. Let me assume that there does exist some very long wavelength radiation, say $100m$ as the lower bound on this. The ratio of temperatures gives that $$ T_{100m}~=~\frac{10^{-3}}{10^2}2.7K~=~2.7\times 10^{-5}K $$ So all this can be is a very cold gas, which is the hitch. This gas is much colder than the background radiation and not in equilibrium. So from some physical grounds this is not likely, and dark matter is most likely not some ordinary form of matter that interacts by EM.

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This is a nice, comprehensible answer. Of course it raises the next question: matter that cannot interact with EM - what on earth ;) do we mean by that? – Gerard Feb 7 '11 at 16:02
Neutrinos don't have em charge, so its not really that strange a concept that dark matter doesnt either. – Virgo Oct 21 '12 at 8:51

There is indeed very good reason to believe dark matter is dark - apart from all the evidence from "missing mass" in luminosity counts and gravitational lensing studies.

This comes from theories of large-scale structure formation: That there has always had to be some sort of matter that doesn't interact electromagnetically at all is crucial to most scenarios of large-scale structure formation. The density fluctuations in the present universe would be too large than what would be predicted if there were only ordinary baryonic matter that interacted only electromagnetically. With dark matter, you can have something that gravitates yet decouples from radiation much before baryonic matter does. This allows the dark matter to form gravitational wells (under collapse) which have a much longer time to expand with the universe. By the time ordinary matter decouples from radiation and joins the rest of the expansion flow, the ordinary matter will quickly fall into these large gravitational wells of the dark matter that have had far more time to grow. This, in a way, amplifies density perturbations in the early universe and allows large-scale structure to form. (to the extent that we see it today in the form of clusters and galaxies)

The required amount of dark matter calculated, in this way, in order to observe the present scale of density fluctuations matches very well with the amount of dark matter required to explain galactic rotation curves, gravitational lensing, etc. So there's excellent agreement that all of these are due to the same thing - some sort of matter which doesn't interact electromagnetically at all, viz. dark matter.

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Aside from the very good theoretical reasons dbrane gives, I'd say that we don't have observational evidence that DM is truly dark. Some of the largely discredited MACHO candidates, like large numbers of red dwarfs, brown dwarfs, planetissimals etc. have been rejected for theoretical -not observational reasons. DM, that is lightly emitting is not currently detectable.

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Nobody is saying that dark matter necessarily doesn't emit or reflect any EM radiation at all. The reason we call it dark matter is that it doesn't emit radiation in the visible spectrum.

The original discovery was simply that the stars in galaxies did not have enough mass to account for the measured mass of the galaxy. Astronomers therefore realized that there had to be some stuff that wasn't emitting visible light, hence they called it "dark." Originally, it was thought that brown dwarfs (very small stars without sustained fusion) could be the dark matter; those wouldn't emit visible light, but they would certainly emit infrared radiation. However, later experiments ruled that out, along with other forms of "normal" (baryonic) matter known to exist.

At this point in time, I believe we know from experimental observations that dark matter is not only dark in the visible spectrum, it appears to emit/reflect negligible amounts of radiation in all other frequency bands that we've checked. But that's just an observation. I don't know of any requirement that dark matter can't interact with EM radiation at all. Most theories about the nature of dark matter do predict that it's totally (or almost totally) decoupled from the EM field. But there is still the possibility that it isn't.

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No radio, UV or X-ray emissions have been found in association with dark matter. – Lawrence B. Crowell Feb 6 '11 at 2:56
I have been told by dark astronomers/astrophysicists mean absolutely no EMR absorption or ambition? – Jonathan. Feb 6 '11 at 10:59

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