Quantum vacuum

The cosmological constant problem is still unsolved. More precisely what is not understood is why quantum corrections due to massive particles such as the electron or phase transition like the change of QCD vacuum around 250 MeV do not give rise to a much larger value of the vacuum energy, which is around 3 times the matter density in the Universe.

Quantum effects are also important as they need to be understood or taking into account when trying to have a fundamental, rather than phenomenological, approach to dark energy.