Q&A: Dark Matter

Q:
Admittedly, dark matter has been inferred from gravitational effects on the unexpected orbital velocities of stars in a galaxy. They do not decrease outwards as predicted by Kepler's Laws. The question is: does the hypothetical dark matter itself rotate? In other words: we "discovered" dark matter from the rotation of stars. How about the rotation of dark matter itself? Does it "matter" that dark matter rotates? ?

A:
The dark matter particles are themselves orbiting in the gravitational well created by the cluster mass, just like the stars do, with approximately the same average velocities as stars -- in this regard, there is no difference between these two matter components. In fact, according to a hypothesis seriously considered until recently, the dark matter might consist of brown dwarf stars. It doesn't matter that the individual DM particles move, since (presumably) they are so numerous that the gravitational potential they collectively create stays the same. However, when big chunks of DM orbit each other (as in a merger of clusters), they entrain the hot intracluster gas and galaxies along with them, so we can observe the signs of this motion.