Calculations show that empty space should bubble with virtual particles, theoretical objects that produce so much energy that they should have blown the universe apart long ago. Since this hasn't happened, theorists had figured that the energy of some of those particles must perfectly cancel the energy of the rest, leaving space as calm as flat soda. Yet observations, based on the brightness of distant exploding stars, show levels of dark energy that are tiny but not zero. If the virtual particles thought to inhabit space are truly the source of dark energy, it's a conundrum: They should produce either huge amounts of dark energy or none at all. "We really, totally are in the dark," says theoretical physicist Lawrence Krauss of Case Western Reserve University.