I read that Dragonfly44 is a galaxy mostly consists of dark matter, and it should contain a super massive black hole too. If there really is a dark SMBH, how does it lose mass? I think there is no way for dark matter to be converted into energy and vice versa, no?
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$\begingroup$ We don't know what dark matter is, so it would be pretty hard to make definitive statements about any of this. The existence of dark galaxies is also unproven. If you mean evaporation by Hawking radiation, then we've no reason to think a "dark SMBH" would be different from a "normal SMBH" or, for that matter, that they would not be different. $\endgroup$– StephenG - Help UkraineCommented Sep 3, 2017 at 12:04
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$\begingroup$ Relevant astronomy.stackexchange.com/questions/11960/… $\endgroup$– James KCommented Sep 3, 2017 at 12:38
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The "no hair" theory of black holes states that a black hole formed from the collapse of normal matter would be exactly the same as a black hole formed from dark matter. You could even form a black hole from photons, if you could get enough of them close together at the same time.
What originally collapsed to form the black hole has no relevance to whether Hawking radiation is given off. A black hole formed of dark matter WIMPS would behave exactly the same as a black hole formed from normal matter.
Dark matter can be converted into "energy" (I'll assume that you mean light, ie photons), in the same way that normal matter can: for example by antimatter annihilation.
The actual formation of black holes, however, requires them to be mostly made of normal matter, since the matter has to lose energy as it falls together, and to do this it must interact with itself (essentially this is friction). As dark matter does not seem to interact with itself or with normal matter, it would not form a major part of a black hole, even in a galaxy dominated by an excess of dark matter. The supermassive black hole may be exceptionally small in such a galaxy, as there is less suitable matter to fall into it.
