In the 1930s, astronomers noticed that the speeds of galactic rotation curves did not go to zero at the edges, as would be required by Kepler’s laws. So around that time Fred Zwicky postulated that there was some unseen matter content in the galaxies which cause them to speed-up. This unseen matter content was called dark matter. But even after 90 years, dark matter has been undetected—there is no agreement even on what exactly constitutes dark matter. Some theories suggest that this is made up of exotic matter beyond the standard model. Others have suggested that this could be some form of heavy neutrinos and so on.