How to Measure the Mass of a Galaxy?

Current how do scientists measure the mass of our galaxy, as it also contains empty space? Let's talk about a galaxy that's far away. It's about how far is it one point five million light years away or so? So first of all, we have to understand how far it is from us. Twere to a good approximation, we know the distance. Now, how do we measure themass of the galaxy? So we see what is the total light output? How much light are we getting in total from the galaxy? That gives us an approximation. Then we look at the rate of rotation of the galaxy. And then based on the amount of red shift and blue shift we