Let me clarify that more with another anology.
Trying to determine how galaxies rotate over the relative short time we've been viewing them is equvilent to looking at pluto then looking at it again 10 seconds later and then look at Mars and look at it again 10 seconds later and then with the only with that tiny bit of information try to explain how all planets and moons orbit.
I think it's done by first finding a galaxy which is appropriately tilted from our point of view, and observing the blueshift of the stars in the arm which is rotating toward us, and the redshift for the part rotating away. If I'm wrong, we'll hear about it soon enough.
However, You would think that if this dark matter stuff can effect the rotation of whole Galaxies if it were present around here it would be effecting the rotation of the planets of the solar system, So unless those perturbsions/anomalies in Uranus and Neptune's orbit that lead to the discovery of Pluto was/is caused by dark matter there isn't any around here.
Personally I think they aren't measuring the rotation of the galaxies accuratly. Considering it takes upwards of 250 million years per rotation and we have only been seriously looking at galaxies for a few decades so we have a lot to learn.
First, I don't think you know how retation curves are measured. The measurements of rotation curves of spirals, ellipticals, and even clusters of galaxies are made with radio spectroscopic measurements of clouds of hydrogen gas. First, in the case of our galaxy, extra care is needed because of the peculiarity of our location inside the disk. Through clever use of trigonometry, it is trivial to measure spectroscopic velocities of objects of known radii within the solar orbit. Note that measuring the redshifts and blue shifts of the doppler shifting of spectral lines gives an instantanous velocity. Astronomers tend to use large clouds because of their large mass. They are hard to gravitationally perturb, and therefore, their velocity is not changed except by the gravitational potential of the galaxy itself.
Outside the solar orbit, it is much different because our geometric relations no longer work. Most early representations of the outer rotation curve used extremely luminous objects to measure the rotation curves, objects like O and B stars, carbon stars, and HII regions. More recently, Merrifield et al (1992) came up with a very clever method of measurement using a relation for the thickness of the HI disk as a function of distance from the galactic center. It is this paper and its followups that lead astronomers to believe that there is dark matter in the galaxy.
For external galaxies, it is advantageous to use edge on galaxies to obtain the rotation curve, or galaxies with a viewing angle of less than 20 degress. Except for a few measurements, nearly every spiral galaxy shows evidence of dark matter.
However, recently, an article was released in Science by Romanowsky, Freeman et al., “A Dearth of Dark Matter in Ordinary Elliptical Galaxies,” Science 301:5640, 1696-1698, 09/19/2003. This paper showed a lack of dark matter as per the rotation curves of the first three elliptical galaxies that they studied in a survey of field elliptical galaxies. It is their belief that this is anomalous because of the location of the galaxies. They were located in sparse clusters, or in clusters without any evidence of intra cluster interactions between members that would have caused the dark matter to be stripped from the cluster. This result is believed by the authors to be important as the main method of formation of ellipticals is through ancient intercluster interaction, where member spirals merge together to create these larger ellipticals. I'm wondering, of course, if this isn't why they don't find much CDM, since any interaction was in the distant past, and the interactions are usually assumed to disperse gas and dust in the elliptical, since most ellipticals contain little or no young stars in them because of the stripping of gas and dust in the past. Perhaps the CDM was stripped in the early interactions? It is tough to say.