So our current overall picture of the Universe is as follows: only around 5 per cent of the energy in the Universe is in the form of normal matter; about 25 per cent is thought to be in the form of the gravitationally attractive dark matter; and the remaining 70 per cent is thought to be in the form of the gravitationally repulsive dark energy. These proportions, give or take a few percentage points here and there, seem suicient to explain all astronomical observations that have been made to date. The total of all three of these types of energy, added together, also seems to be just the right amount to make space flat (rather than positively or negatively curved, like a sphere or a saddle, as illustrated in Figure 10). The flat Universe, filled with mostly dark energy and dark matter, is usually referred to as the Concordance Model of the Universe. Among astronomers, it is now the consensus view that this is the model of the Universe that best fits their data.
The Concordance Model, and all of the observations that have led to it, is undoubtedly a great achievement of 21st-century physics. However, it is certainly not the end of the story. Not with regard to our understanding of the history of the Universe, nor with regards to the way we understand its contents, or the gravitational fields within it. To be blunt, the Concordance Model has a number of shortcomings. First, it appears to have started of in a particularly special configuration. For space to be so close to flat, and to have the background radiation and the distribution of galaxies look so evenly spread, the early Universe needs to have been extremely close to perfectly uniform in density. Second, some of the ripples we see in the CMB appear to be larger than the distance light could have travelled since the Big Bang. Nothing should travel faster than light, in Einstein’s theory, so this is genuinely puzzling. Third, we have no idea what dark matter really is. We only know that it should gravitate, and that it should not interact with light. It doesn’t show up in the Standard Model of particle physics, which has a place for every other known type of particle, and it hasn’t yet been seen in any particle physics experiment. Fourth, the existence of dark energy, and its repulsive gravitational field, seems to require enormous fine-tuning in order to have the efect that we see today. A bit more of it and galaxies would never have formed. A bit less and we wouldn’t have ever noticed it at all.