"Now some people say, "Well, if there's virtual particles there it's really not nothing," but there are no real particles. You try and measure things there, there's nothing, but those virtual particles can give space energy and in fact we've discovered to our great surprise—it won the Nobel prize two years ago—that empty space has energy, and if you put energy in empty space, then it's really strange because it's not like the normal energy... it's not gravitationally attractive, it's actually repulsive, and we've discovered the expansion of the universe is not slowing down like any sensible universe should do. It's actually speeding up... because it's dominated by the energy of empty space."