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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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.
