Universe's dark energy expansion, illustration. The grid of space-time shows the universe and its galaxies, with several distances labelled in light years (ly). The distances shown here are tens of millions of light years. It was originally thought that the expansion of the universe was slowing, but observations of distant supernovae showed that the universe had been expanding at a slower rate in the past. It is thought that the accelerating expansion is being driven by an unknown quantity called dark energy. Image published in 2016.