  Neutron stars are attractive places to look for dark matter because their
high densities allow repeated interactions. Weakly interacting massive
particles (WIMPs) may scatter efficiently in the core or in the crust of a
neutron star. In this paper we focus on WIMP contributions to transport
properties, such as shear viscosity or thermal conductivity, because these can
be greatly enhanced by long mean free paths. We speculate that WIMPs increase
the shear viscosity of neutron star matter and help stabilize r-mode
oscillations. These are collective oscillations where the restoring force is
the Coriolis force. At present r-modes are thought to be unstable in many
observed rapidly rotating stars. If WIMPs stabilize the r-modes, this would
allow neutron stars to spin rapidly. This likely requires WIMP-nucleon cross
sections near present experimental limits and an appropriate density of WIMPs
in neutron stars.
