Although some theories, such as that of cold dark matter, are quite successful in explaining certain aspects of the formation of structure, we seem not to approach a satisfactory theory which can easily account for all the observational constraints on all scales. Most difficult to explain are the indicated clustering of clusters and bulk velocities on very large scales, when considered together with the structure on galactic scales and the isotropy of the microwave background. If these observations are correct, the only scenarios that can work are hybrids of certain sorts, which involve somewhat ad hoc choices of parameters; they are not the theories that would have emerged naturally from first principles, and they do not satisfy the criteria of simplicity and elegancy. I will discuss the currently popular scenarios and the apparent difficulties they face.
Large-Scale Structure in the Universe: Spatial Distribution and Peculiar Velocities
