What is done is:
Calculate the rotational velocity at a specified distance of a galaxy, or this galaxy, by the frequecy shift of stars in their spectra, to determine if they are approaching or receding, and in what basic direction.
From that rotational velocity, it can be calculated what the average speed should be at a specific radius from the galactic center, based on the gravitational constant, andorbital mechanics.
Since these speeds are faster than they should be, it is inferred that there is more mass available in the galaxy than can be seen, based on stellar density, which makes the whole shebang rotate faster.
Part of it is likely the Black Hole at galactic center in Sagittarius, but farther out in the spiral arms, this effect is diminished, thus leading to the search for more "Unseen" Mass.
Very similar to the way the outer planets of our solar system were found.
Unless of course, this is not what you meant.
