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Through the time- and space-resolved interferometry of a short-pulse low-energy probe beam reflecting on the rear surface of a solid target irradiated on its front surface by a high-intensity laser, we have measured a very abrupt expansion of the target rear surface. The experiments were performed using the LULI 100-TW laser facility with a maximum of 10-20 J energy pulses of > 1019 W ldr cm-2 intensity, wavelength of 1.053 mum irradiating Al targets. The detected phase changes, with a few micrometers spatial resolution and picosecond temporal resolution, are interpreted as induced by the cloud of fast electrons having propagated through the target and expanding into vacuum. The measurements have been performed using a laser-pulse duration of 320 fs and a laser energy of 20 J, varying the target thickness from 25, 14, and 9.4 mum. The experimental phase measurements are compared to simulations obtained by post-processing simulation data, run with a 1-D adiabatic plasma-expansion code. The comparison allows one to access, for the first time, to the dynamics of the density and temperature of laser-accelerated fast electrons in solid targets and expanding into vacuum. The same technique also allows one to have information regarding the cold electrons and the energy-partition dynamics.