ESA/Hubble & NASA, R. Tully; ack I’VE been giving a lot of talks about my research to a range of scientific audiences of late. The listeners range from groups that are mainly undergraduates to those made up of specialists in my field – in other words, people who are also searching for and trying to understand the behaviour of dark matter. In nearly all of these presentations, I start by explaining Vera Rubin and Kent Ford’s observations of galaxies. These showed that there was a mismatch between their measurements of galactic masses and what one might expect the mass to be …