We only discovered the Universe’s major mass component, the thing that makes up 70% of it, in 1998. We call it dark energy – although nobody has the slightest idea what exactly it is.

Only about 4.9% of the mass-energy of the Universe is atoms: the kind of stuff you, me, the stars and galaxies are made of (and, of that, only half has been spotted with telescopes). About 26.8% of cosmic mass-energy is invisible dark matter, revealed because it tugs with its gravity on the visible stuff. Candidates for what makes up dark matter include hitherto unknown subatomic particles and black holes made in the Big Bang. But, in addition to dark matter there is dark energy, accounting for 68.3% of the mass-energy of the Universe.