The constituents of the universe
From observations of stellar and galactic motions, astronomers know the universe must contain around five times as much dark matter as ordinary visible matter. Adding dark energy to the picture is a little harder. It isn’t made up of material particles, as dark matter presumably is, so we can’t simply characterise its contribution as so many kilograms per cubic metre. But thanks to Einstein’s theory of relativity, we know that energy is equivalent to mass, and cosmological observations allow us to work out the amount of dark energy in a way that is directly comparable to the other two. The result, according to NASA’s latest estimate, is that the universe is 68 per cent dark energy, 27 per cent dark matter and just five per cent ordinary matter.