This diagram shows the distribution of matter and energy in the Universe. Matter familiar to us on Earth - the atoms and molecules that make up everything we see - account for only a small fraction of the total mass-energy of the Universe.

An unknown form of matter, dubbed "dark matter" accounts for five times more mass in the Universe than ordinary matter, and fills 20% of the mass-energy of the Universe.

The remainder, called "dark energy" by astronomers, comprises 76% of the mass-energy of the Universe. This dark energy is responsible for the recent acceleration in the expansion of the Universe.