Dark energy is a medium of neutral particles whose mass is ten or more times that of visible matter. It makes the universe decelerate its expansion.
Aaron,

Vacuum energy is responsible for dark energy, so the scientists speculate, which in turn makes the universe accelerate its expansion.

The energy of the vacuum comes from virtual particles that have fleeting existence in matter/anti-matter pairs before they recombine. Hawkings black hole radiation comes from virtual pair production at the event horizon, one member of the pair being absorbed by the black hole and the other radiated away.

Vacuum energy can be measured in the lab where it is called the Casmuir Effect.

^That's all I know abvout it.

yanniru