Vacuum

More than sixty years after the early years of quantum field theory -QFT-, the concept of vacuum is still an extreme challenge!. Exprementally, QFT is remarkably successful unless it comes to the physics of its vacuum. Although the pillars of the Fock space are defined based on a vacuum, we can still enjoy the safe side of physics by constraining ourselves not to deal with gravity. For instance, physicists are proud of the unbelievable success of standard model of particle physics in the explanation of data in colliders.
Nevertheless when it comes to include gravity, it is just a mess!!. The tricks we usually use for the normalization of vacuum energy in flat space-time -the so called normal ordering- to throw away the infinite energy of vacuum looks like a joke!. That infinite amount of energy is coupled to the gravity, we can't get rid of it using the normal ordering game or any other complicated known method in QFT.
On the other side, generically, the symmetries of Riemannian space-time may not be enough to define a universal vacuum for the theory.
These two together with tens of other problems of curved space-time QFT together with the disaster of non-renormalizability of quantum gravity describe an opaque picture of the universe. Whereas, there are robust experimental data for the existence of a massive vacuum -cosmological constant- in cosmosology.
Someone should come up with a brand-new idea.