Astronomy is the most awe-inspiring of the sciences. To look up on a truly dark night, away from the light pollution of civilisation, is to be struck by the sheer amount of stuff that makes up creation. But modern astronomy teaches that the stuff that can be seen with the naked eye isn't even the half of it. The matter that can be seen spread across the sky as planets, comets, stars, nebulae, galaxies and the rest makes up just under 15% of the total. Astronomers have dubbed the remaining 85% “dark matter”, because it neither absorbs nor gives off light. Although they are pretty sure it is real, they know nothing about it directly. How can scientists be so sure that something that is invisible is nevertheless there?
