We can model the motions of planets in the Solar System quite accurately using Newton's laws of physics. But in the early 1970s, scientists noticed that this didn't work for disc galaxies - stars at their outer edges, far from the gravitational force of all the matter at their centre - were moving much faster than Newton's theory predicted.

Matter is what makes up the universe, but what makes up matter? This question has long been tricky for those who think about it - especially for the physicists. Reflecting recent trends in physics, my colleague Jeffrey Eischen and I have described an updated way to think about matter.