To determine mass of a galaxy, we apply the same laws (Kepler's and
Newton's) used to determine the mass of the Sun, binary stars, and
our own galaxy. We look at the velocity (or ROTATION rate) of gas or stars, as a
function of radius for a galaxy. The speed of the stars tells us
how much mass it is in orbit around. High speed = lots of mass.
Rotation Curves
What has been found in both ours and other galaxies is that stars are
orbiting much faster than they should. The stars appear to be
responding to a LOT more mass than we can see. Gravity does not lie. The
matter IS THERE. Its just not emitting light. This material, whatever
it is, is referred to as Dark Matter.