All the matter and light we can see in the universe makes up a trivial 5 per cent of everything. The rest is hidden. This could be the biggest puzzle that science has ever faced. Since the 1970s, astronomers have been aware that galaxies have far too little matter in them to account for the way they spin around: they should fly apart, but something concealed holds them together. That 'something' is dark matter - invisible material in five times the quantity of the familiar stuff of stars and planets. By the 1990s we also knew that the expansion of the universe was accelerating. Something, named dark energy, is pushing it to expand faster and faster. Across the universe, this requires enough energy that the equivalent mass would be nearly fourteen times greater than all the visible material in existence. Brian Clegg explains this major conundrum in modern science and looks at how scientists are beginning to find solutions to it. Brian Clegg is the editor of popularscience.co.uk and the author of many books, including most recently Professor Maxwell's Duplicitous Demon (Icon), Conundrum (Icon) and Are Numbers Real? (St Martin's Press). His Dice World and A Brief History of Infinity were both longlisted for the Royal Society Prize for Science Books. Brian has written for numerous publications including The Times, the Observer, the Wall Street Journal, BBC Focus and Nature.