Transcript: Newton’s law of gravity gives astronomers a way of estimating the mass of something from the motions of objects within it. In the solar system or when an object has its mass concentrated in the center, the circular velocity declines with increasing distance from the center going as one over the square root of the distance. This is the characteristic of Keplerian orbits, but in the Milky Way there’s a flat rotation curve which means that the velocity is not declining but it’s flat or even rising with increasing distance. In this same formalism that means that the mass of the galaxy, estimated at different radii, would continue to increase. The high speed of stars in orbits in the disk of the galaxy is apparently driven by a large amount of mass in an extended halo of the Milky Way.