The rst evidence for dark matter, discovered in the late 1960s by American astronomer Vera Rubin, is What are called galactic rotation curves, that is, the orbital rotation velocities of stars as a function of radial distance from the centers of spiral galaxies, such as the Milky Way. So, let's study this phenomenon and understand it. Assume that the density, p, of the galaxy is uniform throughout the galactic disk of radius, R, and thickness, t, which is very small compared with R. a. Write an expression for the mass, M, inside a radius, r ( R. Find a formula for their expected orbital speed. e. Draw a rough plot of your results for v vs. r for r ranging from O to SR