JANUS 16 : Why cosmic expansion accelerates. Jean-Pierre PetitHow the cosmological model Janus explains the acceleration of cosmic expansion, and poses as an alternative to the black energy of the standard model. The acceleration of expansion is due to the negative pressure of the invisible negative mass content of the universe. The nature of these constituents of negative energy is described, and identifies in passing the nature of the mysterious dark matter and primordial antimatter, which is largely absent from cosmology.