Dark energy is the largest and most perplexing component of the universe.

Dark energy is the largest and most perplexing component of the universe. The simplest possibility, that it is the energy associated with quantum vacuum uctuations, suffers from the fact that calculating how much quantum nothingness weighs has eluded theorists for more than ve decades and naive estimates are at least 55 orders of magnitude too large! This suggests to some that ultimately it will be shown that even quantum nothingness weighs nothing, because that outcome seems more likely than nding a means to reduce the naive estimate by precisely 55 orders of magnitude or more. Yet, if the dark energy is not quantum vacuum energy, what is it?