Universe's dark energy expansion. Illustration of the relative distances and geometry between three distant points in the universe, shown as a triangle on an expanding grid of space-time. The distances are marked in light years. The visible matter (the Universe's luminous stars) is not enough to explain the universe's observed expansion rate. This missing mass can be accounted for by non-luminous objects and massive particles, as well as dark energy, a relatively unknown quantity that is thought to be driving the Universe's expansion. Illustration published in 2016.