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so if any kind of 'matter' radiates why doesn't dark matter?

i mean what kind of interaction has dark matter? only with gravity and gravitational waves ? or what?

is it supposed that dark matter and dark energy are related by einstein's equation $ E=mc^2 $

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closed as off-topic by StephenG, John Rennie, stafusa, Kyle Kanos, Chris Mar 8 '18 at 1:07

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  • $\begingroup$ 1) No, only charged matter radiates, and we think dark matter is uncharged precisely because we don't see radiation from it. 2) Nobody knows! Depends on the model you use. 3) That holds for dark matter, and dark energy, and everything else. $\endgroup$ – knzhou Mar 7 '18 at 15:57
  • $\begingroup$ These are all questions answered on e.g. Wikipedia and a simple web search would answers them, and the "related" list that SE generated has several questions that would answer your query as well. $\endgroup$ – StephenG Mar 7 '18 at 16:04
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This is a serious answer: We don't know.

For the moment we can measure its gravitational effects and we can discard most of the elementary particles we know so far. We may only speculate.

What kind of matter radiates: particles that interact with the electromagnetic force (electrically charged matter).

To my knowledge, there is no clear relation between dark matter and dark energy so far.

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Dark matter and dark energy have very little to do with each other, besides the fact that we don't know what either of them are made of.

The types of interactions that dark matter is allowed to have are, perhaps unsurprisingly, heavily dependent on which particular model you use to describe it. In general, though, we expect it to do at least one of three things:

  1. Decay into standard-model particles at an extremely slow rate;

  2. Transfer momentum when it collides with a nucleus, where the cross-section for collision is extremely small; or

  3. Be produced from high-energy collisions of standard-model particles at extremely low rates.

There are exceptions to this list (probably the most prominent exception is the axion, which turns into a photon when it's put into a very high magnetic field), but the above three characterize the vast majority of dark matter detection experiments currently underway. These can all be summarized in one statement: we have to assume that dark matter has at least some tiny coupling to ordinary matter.

It's certainly possible that no such coupling exists, and dark matter exists in complete isolation from ordinary matter, but then we would be completely out of luck as to determining its properties (not that its properties would matter much at that point anyway; in that case, the only way it would produce observable effects is in its gravitational effects, which can be understood independent of its composition).

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Dark matter is assumed to have positive energy density, but negligible pressure. “Dark energy” is the popular term for Einstein’s cosmological constant, which augments the stress-energy tensor of matter and radiation as ${{T}_{\mu \nu }}+\Lambda {{g}_{\mu \nu }}$, so its energy density and pressure must be equal and opposite. The numerical values of energy density and pressure are derived from fits to cosmological data, so you can take these statements as the working definitions of two things we don’t understand.

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