However, the naıve assumption that stellar mass follows halo mass, leads to too many small galaxies, too many big galaxies in the nearby universe, too few massive galaxies at high redshift, and too
many baryons within the galaxy halos. In addition there are structural problems: for example, massive galaxies with thin disks and/or without bulges are missing, and the concentration and cuspiness of cold
dark matter is found to be excessive in barred galaxies and in dwarfs. The resolution to all of these difficulties must lie in feedback. There are various flavours of feedback that span the range of processes
including reionisation at very high redshift, supernova (SN) explosions, tidal stripping and input from active galactic nuclei. All of these effects no doubt have a role, but we shall see that what is missing
is a robust theory of star formation as well as adequate numerical resolution to properly model the interactions between baryons, dynamics and dark matter.