What measurements of the mass of the milky way indicate?

Answer 1

That we need better theories and measurements. We cannot directly measure the mass of anything we can't put on a scale in a laboratory. What we can do is measure things like the light and other electromagnetic radiation coming at us from other objects in our direct line of sight, and apply theoretical computation and a few good guesses to get an approximation of what the mass of that object is. Taking into consideration that there are millions of objects in our milky way, and that more than half of them are not in our direct line of sight, these approximations change quite a bit when we apply different theories to what we have measured. Also, we are getting better tools to measure with, like better telescopes, detecting a wider range of electromagnetic radiation, putting some of these tools on sattelites so we get a better view, and developing more workable theories to help us explain the universe around us based on these measurements.