I found it a bit hard to understand before I actually took the course in university, but I'll try to explain simply: You can infer the mass that is interior to

I found it a bit hard to understand before I actually took the course in university, but I'll try to explain simply: You can infer the mass that is interior to an orbit by measuring the speed at which an object is orbiting. By measuring the redshift and blueshift, we can get speeds of different parts of the galaxy. It turns out that we can account for more-or-less all of the mass near the centre of galaxies, but as we get further out, orbital velocities seem to be higher than they should be implying more mass is there than we can actually see. In other words, we can tell that the missing mass is not at the centre of the galaxy, but rather spread throughout it.