Quantum physics, and the widely accepted and experimentally tested Heisenberg Uncertainty Principle implies that no 'vacuum' can be truly empty. At all times, innumerable quantised 'virtual particles' must be coming into existence and then disappearing. These particle must have a collective 'mass' - or equivalent energy. Calculations regarding the Universe's 'vacuum energy' vary enormously.
According to calculations that take into account the Cosmological Constant, the energy should be around 10−9 joules per cubic meter. If on the other hand the calculation is based around Quantum Electrodynamics and the Planck Constant, the figure is 10113 joules per cubic meter.
One (or both) of the calculations must be wrong - with an error margin of more than 120 orders of magnitude - making it one of the most spectacular mathematical disagreements in all of physics.
The lack of agreement has profound implications for theories of gravity, dark energy, dark matter, black holes etc etc.
Also see: Cosmological Constant Problem