As we have learned, the Milky Way is a huge galaxy, containing about 100 billion stars. The mass of the Milky Way inferred from the amount of visible matter is about 750 billion - 1 trillion times the mass of the Sun, where the mass of the Sun is about 2 x 1030 kilograms.
However, if we calculate the mass using a different technique based on the dynamics of stars in the Galaxy, we derive a different number. The stars in the outer parts of the Galaxy are rotating faster than they should be compared to expectations from the amount of visible matter. This implies that the mass of the Milky Way is some ten times more massive than expected, and this extra or "missing" mass is known as dark matter.
The rotation curve of the Milky Way, which is the rotational velocity of stars as a function of distance from the center of the Galaxy, is shown below.
We still do not know the nature or exact amount of dark matter in the Milky Way. Theories about the nature of dark matter include everything from the size of subatomic particles (called WIMPS, or Weakly Interacting Massive Particles) to failed, dark stars (called MACHOs, or MAssive Compact Halo Objects). Experiments to detect enough MACHOS to account for the missing mass have failed, but neither theory has yet been ruled out.
With SIM PlanetQuest, we will be able to determine the size and the distribution of mass in the Galaxy, as well as the kinematics of stars in the outer halo. These properties will enable us to better understand the amount, distribution, and possibly the nature of the dark matter in the Milky Way.