Reflections on Cosmology: an Outsider's Point of View

Some of the assumptions of cosmology, as based on the simplest version of General Relativity, are discussed. It is argued that by slight modifications of standard gravitation theory, our notion of the sources of gravity {the right hand side of Einstein's equations{, could be something radically different from what is usually expected. One example is exhibited to prove the point, and some consequences are discussed.