  The fact that dark matter (DM), thus far, has revealed itself only on scales
of galaxies and larger, again thrusts onto astrophysics the opportunity and the
responsibility to confront the age old mystery "What is the nature of matter?"
By deriving basic data on the nature of DM - e.g., mass of its particle(s),
present mean temperature, distribution in galaxies and other structures in the
universe, and capacity for dissipational collapse - we will be uncovering the
properties of the dominant species of matter in the universe and significantly
extending the standard models of particle physics. Determining the mass of the
DM particle to an order of magnitude would help to sort out the particle family
to which it (or they) belongs. Beyond mass, there are issues of stability. The
DM particle may be unstable with a measurable half-life, or it may become
unstable after absorbing a certain amount of energy from collisions. In both
cases it would contribute to the present hot dark matter component.
  Some key parameters of DM can most accurately be measured in the very nearby
universe because DM dominates the mass in the outer Milky Way (MW), in other
galaxies in the Local Group, and in the Local Group in its entirety. The
presence and amount of DM can be quantified by study of dynamical processes
observable in fine detail within these entities. Precise measurements of 3-D
velocities for stars, coherent star streams, and stars in satellite stellar
systems out to the edge of the Galaxy can reveal "what is the shape,
orientation, density law, and lumpiness of the dark matter halo" as well as
"what is the total mass of the Galaxy?"
