Dark energy
Dark energy is the term for a proposed form of energy that is thought to permeate all space. Dark energy has a repulsive effect, much weaker than gravity, but falling off in strength over distance much less rapidly. The true nature of dark energy is one of the current unsolved problems in physics.
The abundance of dark matter is 23% of the total mass of the universe, which is far larger than the 3.6% of mass represented by visible and non-visible normal matter, such as interstellar gas, and 0.4% of mass represented by visible stars. But 73% of the mass of the universe is bound up in the mass-equivalence of dark energy.
[edit] Evidence
Not only is the universe expanding, its rate of expansion is increasing with time. Dark energy is the name given to the mysterious "something" that is fuelling this universal inflation. Such energy would also add to the observed flatness of the universe.
Dark energy is the successor to such past scientific gap-fillers as caloric, phlogiston, aether and Einstein's "biggest blunder" - the cosmological constant. Whether it stays as it is, becomes modified into a better-accepted form, or is replaced by an even stranger spacefiller remains to be seen.