This article shows that the vanishing covariant divergence of the energy-momentum tensor of matter is a conservation law. In addition, it is pointed out why energy-momentum pseudo-tensors of the gravitational field cannot represent the energy density of the gravitational field, but, apart from a factor, is described by the Einstein tensor. The necessarily existing conservation law of total energy, momentum and stress in general relativity is derived, thereby solving the cosmological constant problem as well as that of dark energy and dark matter. In Newton’s theory of gravitation, it is the modified Poisson equation that approximately meets the requirement of conservation of total energy. A simple but fairly accurate model, consistent with observations, is presented, solving the modified Poisson equation to fit the calculated rotation curves to the observed speeds in spiral galaxies being composed of several components: the central region of the bulge, the bulge, the disk and dark matter, the latter of which is exactly described by this model.