I know this isn't about observing, but it is about deep sky and I thought you might appreciate hearing about this. Several American astronomers claim that when they observe galaxies with IR they can account for all observed rotation velocities by only taking into account visible matter. For more details, see
Apparently this needs to be taken with a grain of salt, or two. The author has been trying to prove no dark matter exists for years now and still hasn't succeeded. Next time I'll try to get some background before I post something like this.
Assuming normal gravity, they haven't found all the mass - just that the total mass is locally proportional to the visible one. We can settle for that.