Viscosity of holographic fluid in the presence of dark matter sector
  Based on the gauge/gravity correspondence, the hydrodynamic response coefficients, shear and Hall viscosities, have been studied. The holographic model of Einstein-Maxwell- AdS $(3+1)$-dimensional system additionally coupled with the another gauge field mimicking the {\it dark matter} sector, as well as, gravitational Chern-Simons term bounded with a dynamical scalar field, were taken into account. Condensation of the scalar field provides the parity violating term. Both shear and Hall viscosities have been calculated and their dependence on $\alpha$ - the coupling constant between matter and {\it dark matter} sectors has been studied. To the lowest order in the derivative expansion and perturbation in $\alpha$, the shear viscosity is not influenced by the {\it dark matter}, while the Hall component linearly depends on $\alpha$. 
Marek Rogatko, Karol.I.Wysokinski
hep-th