In a universe dominated by Dark Matter and Dark Energy, galaxy formation is a complex combination of hierarchical clustering, gas dissipation, merging events and secular evolution. Numerical simulations are one of the most powerful tools to follow the assembly and evolution of galaxies in such a dark universe. I will first briefly introduce the methods used in numerical cosmology and I will then present the NiHAO project, a large simulation campaign led by my research group at the MPIA. Thanks to a combination of detailed description of gas and stellar physics and high numerical resolution we have been able to successfully reproduce the properties of observed galaxies (at low and intermediate mass scales), including the solution of the long standing problem of the distribution of Dark Matter at the center of galaxies. I will then present the challenges of forming realistic massive galaxies and our road map to overcome them.