Brian Greene tonite NOVA. "dark energy has turned our picture of the cosmos upside down. And we don't know what it is." Speak for your self, Brian. I know what it is - incoherent virtual bosons whose density inside the vacuum is greater than the density of incoherent virtual fermion-antifermion pairs. The former gravitationally repel, the latter gravitationally attract according to battle-tested mainstream physics, i.e. Lorentz invariance and the equivalence principle.