A pressureless scenario for the Dark Matter ( DM ) fluid is a widely adopted hypothesis , despite the absence of a direct observational evidence . According to general relativity , the total mass-energy content of a system shapes the gravitational potential well , but different test particles perceive this potential in different ways depending on their properties . Cluster galaxy velocities , being \ll c , depend solely on the gravitational potential , whereas photon trajectories reflect the contributions from the gravitational potential plus a relativistic-pressure term that depends on the cluster mass . We exploit this phenomenon to constrain the Equation of State ( EoS ) parameter of the fluid , primarily DM , contained in galaxy clusters . We use the complementary information provided by the kinematic and lensing mass profiles of the galaxy cluster MACS 1206.2-0847 at z = 0.44 , as obtained in an extensive imaging and spectroscopic campaign within the CLASH survey . The unprecedented high quality of our data-set and the properties of this cluster are well suited to determine the EoS parameter of the cluster fluid . Since baryons contribute at most 15 \% to the total mass in clusters and their pressure is negligible , the EoS parameter we derive describes the behavior of the DM fluid . We obtain the most stringent constraint on the DM EoS parameter to date , w = ( p _ { r } +2 p _ { t } ) / ( 3 c ^ { 2 } \rho ) = 0.00 \pm 0.15 \mathrm { ( stat ) } \pm 0.08 \mathrm { ( % syst ) } , averaged over the radial range 0.5 \mathrm { Mpc } \leq r \leq r _ { 200 } , where p _ { r } and p _ { t } are the radial and tangential pressure , and \rho is the density . We plan to further improve our constraint by applying the same procedure to all clusters from the ongoing CLASH-VLT program .