How to hide a 'fifth force' – and how to find one

Several big problems with physics at the moment involve gravity. But because Einstein's theory works so well, it's very difficult to change it. Some recent ideas show a possible way forward

Einstein's General Relativity provides an elegant description of how space, time and matter affect one another. It makes precise predictions of gravitational effects, which have been verified by many measurements.

But if we use the theory to try to understand the motion of galaxies, we get the wrong answer, unless we invent a new form of so-called 'dark' matter. This is not a small correction - there needs to be much more of the Dark Matter than normal matter, and what is more, it doesn't seem to be made up of quarks and electrons like all other matter. In fact it doesn't seem to be made up of any of the particles in the Standard Model of particle physics.