This paper shows that a simple and relativistic extension of Newtonian gravity leads to predictions that fits super- nova observations of magnitude versus redshift very well without having to rely on the hypothesis of dark energy. In order to test the concept, we look at 580 supernova data points from the Union2 database. Some relativistic extensions of Newtonian gravity have been investigated in the past, but we have reason to believe the efforts were rejected prematurely, before their full potential was investigated. Our model suggests that mass, as related to gravity, is also affected by standard relativistic velocity effects, something that is not the case in standard gravity theory, and this adjustment gives su- pernova predictions that fit the observations. Our find- ings are reflected in several recent research papers that follow the same approach; that work will also will be dis- cussed in this paper.