This work investigates the nature of the empty space and of the energy accelerating expansion of the universe, within the context of the Higgs theory. It is consensus among the cosmologists that dark energy, accelerating the expansion of the universe, is energy of the empty space (vacuum) itself. According to the Higgs theory, empty space (vacuum) is filled up by a real quantum fluid medium, closely analogous to the superconducting condensate, giving mass to the elementary particles by the Higgs mechanism. This spatial medium is the holder of the vacuum energy. Current theories describe the empty space (vacuum) in terms of the stress-energy tensor of a perfect fluid and estimate the vacuum energy density in terms of zero-point energies of the various force fields. They come to the scandalous conclusion that the vacuum energy density is 120 decimal orders of magnitude larger than shown by the observations. In the context of the Higgs theory, empty space, far from a perfect fluid, is a very strongly correlated boson condensate, a perfect quantum fluid ruled by the principles of quantum physics and governed by a powerful order parameter. This order parameter is stabilized by a huge energy gap that, according to the Glashow-Weinberg-Salam electroweak model, achieves more than 200 GeV. This huge energy gap very strongly suppresses the quantum fluctuations and the zero-point energies. This lets clear that estimating the vacuum energy density in terms of the zero-point energies cannot be correct. The expanding universe does not create more and more vacuum energy and does not expand against a negative pressure. The universe is an adiabatic system that conserves the total mass-energy and expansion only reduces the vacuum energy density. Calculations within this context show that the vacuum energy density converges closely to the observed value.