If astronomers have concluded that the rotation speed of objects held firmly within a galaxy but far from a galactic center is too great to be explained by the visible matter of the galaxy alone, what is it that makes them certain that an under-calculation of the mass of the galaxy's supermassive black hole isn't a contributing factor, insomuch as galactic black hole masses are calculated partially based on observations of the motions and velocities of visible galactic objects?
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$\begingroup$ The total baryonic (normal-matter) mass within the Milky Way is several tens of billions of solar masses, while the necessary dark matter mass is hundreds of billions of solar masses. The central SMBH has a very well measured mass of slightly more than 4 million solar masses. So even if we were underestimating the SMBH by a factor of a hundred, it would make no difference at all for explaining the rotation of objects located tens of thousands of light years from the center. $\endgroup$– Peter ErwinCommented Aug 13 at 12:03
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$\begingroup$ Thank you Peter. Can you answer a question please? Galaxies rotate and stars orbit the center with the galactic rotation, but in the evolution of a star, when it transitions to a stellar black hole, does it's original stellar orbit change? Thank you. $\endgroup$– mdswartzCommented Aug 14 at 22:51
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$\begingroup$ It's certainly possible, in that asymmetries in supernova explosions can give the resulting remnant a "kick", which will alter their orbit. This is pretty clearly true for neutron stars, and there's some evidence for black holes as well. $\endgroup$– Peter ErwinCommented Aug 15 at 12:38
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It's a question of magnitudes. The density of dark matter required to explain the rotation curves of a galaxy is far too low to make a significant contribution to the mass pulling on stars and gas close to a central black hole.
To put it another way there probably is dark matter contributing to the mass that is attributed to a central, supermassive black hole, but it is negligible.
