From the Washington Post,
"Even nothing, even empty space, weighs something, and because in our universe we've got a lot of nothing, it has a major effect on our evolution and causes space itself to accelerate," said David Spergel, an astrophysicist at Princeton University.
What remains unclear is what dark energy is, exactly.
Nothing weighs something? What is this, a love song? Magic? Voodoo? Inferential science? Ontological epistomology? Whatever it is, it sounds a lot like macroeconomics to me.