Following Newton's new law of universal gravitation an experiment was devised to measure the faint gravitational attraction between two lead balls Wikipedia Cavendish experiment. This was a way of 'weighing the world'. The mass of the Sun could then be calculated from the value of the mass of the Earth. The mass of any star could then be measured by comparing it to the mass of the Sun. But there was an initial logical assumption made; that the nature of the matter comprising the entirety of the Earth is the same as that of the lead balls. This is not necessarily the case. The concept of entropy was unknown in the past. It is possible that the matter at the center of the Earth has a lower entropy compared to surface baryonic matter, for example. This would imply that the calculated density of Earth, 5.5 times that of water, is an overestimate. The mass of the Sun could therefore be an overestimate etc. Is it possible that there simply isn't a Missing Mass Problem after all? Wikipedia Dark Matter.