Abstract : This thesis presents a new method to measure the temperature of ultracold atoms from the spatial autocorrelation function of the atomic wavepackets. We thus determine the temperature of metastable helium 4 atoms cooled by velocity selective dark resonance, a method known to cool the atoms below the temperature related to the emission or the absorption of a single photon by an atom at rest, namely the recoil temperature. This cooling mechanism prepares each atom in a coherent superposition of two wavepackets with opposite mean momenta, which are initially superimposed and then drift apart. By measuring the temporal decay of their overlap, we have access to the Fourier transform of the momentum distribution of the atoms. Using this method, we can measure temperatures as low as 5 nK, 800 times smaller than the recoil temperature. Moreover we study in detail the exact shape of the momentum distribution and compare the experimental results with two different theoretical approaches a quantum Monte Carlo simulation and an analytical model based on Lévy statistics. We compare the calculated line shape with the one deduced from simulations, and each theoretical model with experimental data. A very good agreement is found with each approach. We thus demonstrate the validity of the statistical model of subrecoil cooling and give the first experimental evidence of some of its characteristics: the absence of steady-state, the self-similarity and the non lorentzian shape of the momentum distribution of the cooled atoms. All these aspects are related to the non ergodicity of subrecoil cooling.