Astronomers have two other methods to weigh the galaxy. The first measures the gravitational mass from the speed of rotation of a galaxy as a function of distance from the center, taking advantage of Newton's Laws to determine the mass around which the stars orbit. The second measures the total light produced by a galaxy; since we know how much light individual stars produce, we can determine how many stars are in the galaxy, and, hence, its luminous mass. The discrepancy between measurements of a galaxy's luminous to measure the mass.The second way is to determine masses is to use our knowledge of physics. All mass is expected to interact gravitationally whether or not we can see it. We should be able to measure the mass with our eyes by their effect.