We investigate several quantum many-body systems using the density matrix projection method. We consider a detailed proof of concept with the harmonic oscillator and apply the method to systems composed of fermions, bosons, and mixtures of bosons and fermions within the framework provided by Helium atoms. Bosons serve to demonstrate most of the features of the method. Complementarily, when considering systems described by anti-symmetrical wavefunctions, introducing the fixed-node approximation is the way we choose to avoid the ubiquitous sign problem. One of the main features of the method is its ability to estimate any property at temperature T = 0 without bias, including biases that come from long-running simulations. In great detail, we discuss pair density matrices that we use to construct the full density operator in coordinate representation. Finally, we investigate 3 He-4 He mixtures, showing the robustness of the density matrix projection method. In particular, we calculate the pair correlation function of equal and opposite spins and accurate values of the 3 He kinetic energy.