Dark Matter shows up in cosmological observations as deviation of velocity of e.g. spiral galaxy arms and so on, so we know it interacts gravitational. We cannot see it, and in the Bullet Cluster you can see no interaction of the Dark Matter like the hydrogenium gas shows up - so we assume it to not interact electromagnetic. That leads up for the search for WIMPS, weakly interacting massive particles. But why do we not assume it to interact strongly?
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2If it did, then it could just be part of nucleons/mesons, no? – ACuriousMind Oct 04 '15 at 13:18
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2Decay of DM would produce neutral pions which would produce photons that we would detect. Also, we would probably produce it at LHC. – Ihle Oct 04 '15 at 13:19
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@lhle: We hope to produce it at LHC, and DM is assumed to be stable as it is assumed to be an elementary particle. – A.Landwonder Oct 04 '15 at 13:56
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@ACuriousMind: Why is there a problem with DM to be part of nucleons/mesons? – A.Landwonder Oct 04 '15 at 13:56
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@A.Landwonder See https://physics.stackexchange.com/questions/153722/why-cant-dark-matter-be-baryonic?rq=1 and links therein for an answer to that. – HDE 226868 Oct 04 '15 at 14:32
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@lhle how do we know it would decay if we don't even know what dark matter is?! – TanMath Oct 04 '15 at 18:52
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The elementary particles of the standard model that interact strongly are the quarks, the purple ones in the plot, the interactions happening with gluons.
Note that all of them are charged, and therefore they interact also with the electromagnetic interaction. They would be seen: protons would interact with electrons in space and give off radiation , neutrons would decay into protons and electrons. Higher strong resonances are unstable and would end up into photons electrons and protons in the end. After all if you do not have hydrogen you also do not have protons.
It is only the weak interacting particles that can be candidates for dark matter.
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