There are some significant questions. One possibility is that a star core collapses to form a black hole so there is an event horizon, but the core is not compacted into zero size. Instead, the core collapses to Planck density (10^96 grams/cm^3) and then rebounds. However, time dilation is so severe near the collapsed core that to an outsider the rebound occurs very slowly. Indeed, the black hole looks stable. You can read about this in several articles on the net about Planck stars. You can also read about quark stars and strange stars that are intermediates between neutron stars and black holes. Some pretty mind blowing stuff there...