Ignoring the overall expansion of the universe
Data of observations suggest that known types of a matter make up only about five percent in the universe. Another 20% of the mass of the universe, which are concentrated in galaxies and clusters of galaxies, are not visible and not expected to consist of known sorts of elementary particles. What stays absolutely open and important to physics is the question of what comprises the remaining three quarters of the total energy of the universe, which is almost not expanding along with the space as we think. Since this energy is not concentrated anywhere and distributed apparently uniformly and isotropically, its most ‘simple’ explanation is that it is inherent in vacuum. One of the several other explanations for this dark energy of the universe might be that the theory of gravity is incomplete or unfair at large distances.