Dark Energy as Defined by Wikipedia

In physical cosmology, dark energy is a hypothetical form of energy that permeates all of space and has strong negative pressure.[1] According to the Theory of Relativity, the effect of such a negative pressure is qualitatively similar to a force acting in opposition to gravity at large scales. Invoking such an effect is currently the most popular method for explaining recent observations that the universe appears to be expanding at an accelerating rate, as well as accounting for a significant portion of the missing mass in the universe.

Interested? Though I don't pretend to understand most of it, I am rivoted! See also:

Evidence of Dark Energy
http://www.nasa.gov/home/hqnews/2006/nov/HQ_06353_Hubble_Dark_Energy.html

Dark Energy Changes the Universe
http://www.nasa.gov/missions/deepspace/f_dark-energy.html

Dark Energy Pushing Galaxies Apart
http://www.space.com/scienceastronomy/astronomy/cosmic_darknrg_020115-1.html

Human Origin and Dark Energy
http://www.space.com/scienceastronomy/mystery_monday_040531.html