What does the rotation curve of the Milky Way Galaxy tell us about our Galaxy’s total mass?

Question

What does the rotation curve of the Milky Way Galaxy tell us about our Galaxy’s total mass?

Answer 1

You know how the planets orbit the sun more slowly, the farther away they are?  Well the stars in the galaxy do the same thing.  The problem of predicting their speeds is more difficult than with the planets because the mass of the galaxy is much more dispersed.  Consequently, the gravitational effects are variable because of the differing amounts of matter as you move away from the center.

What they do is measure the speed of stars believed to be at certain distances from the center.  Then they graph a line representing those speeds, from the center out to the edge.  This would be the gravitational curve of a galaxy.  Of course it will start high and come down, but it is usually not a perfect curve.  It serves as a kind of gravitational fingerprint for the particular galaxy.

Edit:  Okay, thumber, I think the rotational curve for our galaxy shows pretty much what we would expect if it had NEITHER a supermassive black hole NOR dark matter.  Maybe a central black hole a hundred or a thousand solar masses.  Not billions.

Answer 2

The simple answer is that the rotation curve of our galaxy strongly implies that there's more mass present than can be accounted for, i.e., "dark matter."