Only 4 per cent of the mass of the Universe is in the atoms that make up you and me, the stars and planets. And only half of that have we ever seen. 23 per cent of the Universe is mysterious invisible, “dark”, matter. We know of its existence only because its gravity tugs on the visible stars. And 73 per cent of the Universe is dark energy. Discovered only in 1998, this invisible stuff fills all of space and it has repulsive gravity. If you can find out what the missing 98% is, a Nobel Prize awaits you!