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Is it possible that dark matter particles are unstable and the existing dark matter will decay in the long run and turn into hydrogen, for example? What would be possible mechanisms for such a decay?

Could some of the hydrogen we observe be "new" hydrogen, i.e. be the product of dark matter decay?

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Yes, as mentioned elsewhere, it is possibly possible. Dark matter particles may be intrinsically unstable (though having long lifetimes, which are at least significantly longer than Hubble time).

Check for more info here:

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Could it be exotic in such a way that it becomes hydrogen, similarly to how hydrogen turns into helium through fusion? Then if ordinary visible matter turns into dark matter somehow, it would be a full circle. – frodeborli Jan 14 '14 at 14:40
There wouldn't be a full circle, due to first and second laws of thermodynamics: ordinary matter would need a lot of energy to transform back into dark matter particles. But DM can create some ordinary matter, hypothetically. – Alexey Bobrick Jan 14 '14 at 14:47
Hmm... Thus requiring it to accumulate energy for billions of years. By the way, DM could be a chemical element at the end of the periodic table "anti radioactive" which then splits to become hydrogen. I just want it to be full circle because it would almost guarantee the creation of life by chance, and enable civilizations that are billions of years old and a lot more, but I'm on very thin ice here... :) – frodeborli Jan 14 '14 at 16:54
What do you mean "..due to dark matter accumulating energy." ? – astromax Jan 14 '14 at 19:12
DM is certainly not made of chemical elements with baryonic composition of the atomic nucleus, no matter how strange. That's ruled out by Planck data. – Gerald Jan 14 '14 at 20:20

Dark matter is actually matter. Real matter, just that we can not observe it.

Part of it is actually cold hydrogen, and even cold dust.

Other parts of it are actually different particles that may decay. But that does not mean formation of new Hydrogen. To form Hydrogen you need both a proton and an electron to get bound together, and this implies they meeting, which is very difficult in low density regions like those of extragalactic dark matter.

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Pure hydrogen and "dust" should be observable, but dark matter is only indirectly observed. – frodeborli Jan 14 '14 at 14:43
Well, protons are ionised hydrogen. And given that IGM is very hot and significantly ionised, protons to contribute to hydrogen. – Alexey Bobrick Jan 14 '14 at 14:44
Dark matter behaves the same way as mass does (gravitationally speaking), but it is absolutely not simply dust and gas that we have a hard time seeing. Dust and gas still interact electromagnetically (that is, emit photons; albeit at lower frequencies if cold). Dark matter appears not to interact electromagnetically. They are completely different categories. – astromax Jan 14 '14 at 19:11
@astromax, in fact, some part of what is called DM can come from our incomplete knowledge of IGM or even from not so easily observable outer parts of spiral galaxies. – Alexey Bobrick Jan 17 '14 at 16:45
From your link: "Both indicate that the amount of baryonic dark matter is much smaller than the total amount of dark matter." Therefore, baryonic dark matter has a very small impact on our understanding of dark matter and while you can mention that there is baryonic matter that we can't observe, it is not the answer to the question. – Aaron Jul 29 '14 at 19:10

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