The current hierarchical theory for galaxy formation — in which larger galaxies are built up from smaller building blocks and dark matter dominates — is well studied and reasonably well accepted. The book is still far from closed, however.
Galaxies come in many different flavors, therefore models of galaxy formation and evolution must reconcile theory with what we observe. The issue addressed in this paper is that current semi-analytic models over predict the fraction of late-type satellite galaxies that are passive (not forming stars).
We often think of galaxies as collections of stars, but this simple picture omits (among other things) the fascinating interstellar medium (ISM)!