  The Universe contains approximately 6 times more dark matter than normal
baryonic matter, and a directly observed fundamental difference between dark
matter and baryons would both be significant for our understanding of dark
matter structures and provide us with information about the basic
characteristics of the dark matter particle. We discuss one distinctive feature
of dark matter structures in equilibrium, namely the property that a local dark
matter temperature may depend on direction. This is in stark contrast to
baryonic gases. We used X-ray observations of two nearby, relaxed galaxy
clusters, under the assumptions of hydrostatic equilibrium and identical dark
matter and gas temperatures in the outer cluster region, to measure this dark
matter temperature anisotropy beta_dm, with non-parametric Monte Carlo methods.
We find that beta_dm is greater than the value predicted for baryonic gases,
beta_gas=0, at more than 3 sigma confidence. The observed value of the
temperature anisotropy is in fair agreement with the results of cosmological
N-body simulations and shows that the equilibration of the dark matter
particles is not governed by local point-like interactions in contrast to
baryonic gases.
