I initially thought of posting this thread in cosmology section but then I realised it's much more general in scope,so Iam posting it here. Massive bodies attract gravitationally and mass-energy equivalence implies that energy contributes to gravitational attraction. Then what's so peculiar about vaccum energy that it gives rise to gravitational repulsion instead of attraction? Is this because of some kind of internal structure it has,which is somehow producing repulsion at the microscopic scale. Or is it a mere fact that has been observed experimentally(expanding universe etc.) and no theoretical justification yet. Isn't the term vaccum energy itself a misnomer?