B. Some caveats

The most important caveat in all of this is the fact that when studying cosmic structure we observe only the luminous constituents of the Universe. It is true that we can observe cosmic structure over an enormous range of the electromagnetic spectrum, but nevertheless we face the prospect that about 85% of what there is out there may forever remain invisible except indirectly though its gravitational influence.

Fortunately, we can directly study the gravitational influence of the dark component in a number of ways. If it is uniformly distributed it has an influence on the overall cosmic expansion and on the physics of the early Universe. We can detect its influence by studying the cosmic expansion law, or by studying the nature of the spatial inhomogeneities seen in the cosmic microwave background radiation. If it is not uniformly distributed it will influence the dynamics of the large scale structure as seen in the velocity maps for large samples of galaxies and it may reveal itself through studies of gravitational lensing.

Our numerical simulations of the evolution of structure can in principle take account of several forms of matter. While this has been a successful program, the lack of detailed knowledge about the nature of the dark matter is nevertheless a serious impediment. Some astrophysicists would turn the problem around and argue that those simulations that best reproduce what is seen will provide important information about the nature of the dark matter.