  The nature of the dominant component of galaxies and clusters remains
unknown. While the astrophysics community supports the cold dark matter (CDM)
paradigm as a clue factor in the current cosmological model, no direct CDM
detections have been performed. Faber and Visser 2006 have suggested a simple
method for measuring the dark matter equation of state that combines kinematic
and gravitational lensing data to test the widely adopted assumption of
pressureless dark matter. Following this formalism, we have measured the dark
matter equation of state for first time using improved techniques. We have
found that the value of the equation of state parameter is consistent with
pressureless dark matter within the errors. Nevertheless, the measured value is
lower than expected because typically the masses determined with lensing are
larger than those obtained through kinematic methods. We have tested our
techniques using simulations and we have also analyzed possible sources of
error that could invalidate or mimic our results. In the light of this result,
we can now suggest that the understanding of the nature of dark matter requires
a complete general relativistic analysis.
