(Only four per cent of the mass of the Universe is in the atoms that make up you and me, the stars and planets. And we’ve only ever seen half of that with our telescopes. 23 per cent of the mass of the Universe is invisible, “dark”, matter. We know of its existence only because its gravity tugs on the visible stars and galaxies. No one knows what it is. And 73 per cent of the mass of the Universe is dark energy. Discovered only in 1998, this invisible stuff fills all of space and it has repulsive gravity. Although I do think that calling gravity repulsive is a bit over the top, gravity simply sucks.)