 The second set of open questions regards structure formation. While the account of structure formation matches several significant observed features, such as the correlations among galaxies in large scale surveys, there are a number of open questions about how galaxies form (Silk 2017). Many of these, such as the cusp-core problem (Weinberg et al. 2015), and the dark halos problem (a great many more small dark halos are predicted around galaxies than observed) regard features of galaxies on relatively small scales, which require detailed modeling of a variety of astrophysical processes over an enormous dynamical range. This is also a very active area of research, driven in particular by a variety of new lines of observational research and large-scale numerical simulations.