IT IS ONE of the biggest mysteries in cosmology—and getting bigger all the time. Ever since Edwin Hubble, an American astronomer, published observations of distant galaxies in 1929, scientists have known that the universe is expanding. For almost 30 years they have known that the expansion is accelerating (that discovery, made in 1998, was honoured with a Nobel prize in 2011). What they cannot agree on, though, is how fast it is currently growing.