Dark energy is a mysterious entity that is hypothesized to be the responsible for the expansion of the universe. The first observational evidence for its existence came from measurements of supernovae, which showed that the universe does not expand at a constant rate; rather, the universe's expansion is accelerating. Other than that, it is a complete mystery. It turns out that roughly 68% of the universe is dark energy.

Dark matter makes up about 27%. The primary evidence for dark matter comes from calculations showing that many galaxies would fly apart, that they would not have formed, or that they would not move as they do if they did not contain a large amount of unseen matter.

The rest - everything on Earth, everything ever observed with all of our instruments, all normal matter and antimatter - add up to less than 5% of the universe.

More Info: science.nasa.gov