Dark matter is invoked in those astronomical scenarios where there is a discrepancy between the baryonic mass and the dynamical mass. Such scenarios include the rotation curves of disk galaxies and the high velocities of galaxies in galaxy clusters. For our alternative explanation we assume there is no dark matter and guess that the dynamical mass is a weighted sum of the baryonic mass. When we do this, it turns out that the required weighting function has a simple power law structure, and this is sufficient to explain both disk galaxies and galaxy clusters. The weighting function also enables us to predict the dynamical mass distribution from the baryonic mass distribution, so we can predict the rotation curves of disk galaxies and the velocities of galaxies in galaxy clusters. This basic observational result is difficult to ignore as it is so simple, and surpasses what dark matter and modified gravity theories can do. We make no use of dark matter and we make no changes to Newtonian gravity.