1.5. Importance of the Dark Matter Problem Today

The dark matter problem is nowadays a complex of problems, which is pervasive in every aspect of extragalactic astronomy and cosmology. This can be gleaned from previous reviews and symposia, e.g. Trimble 1987, Kormendy & Knapp 1987, Holt & Bennett 1995, Zaritsky 1998). Another way of seeing this is to look at the way the apparently simple problem of the origin of the Hubble sequence has evolved. The classic view : ellipticals and bulges form quickly, and spirals disks are built up gradually, still has its supporters (e.g. Sandage 1986). However, new ideas about the importance of secular evolution modifying the Hubble type of spirals, and producing ellipticals from merging disk galaxies (Toomre 1977), are now used together with numerical simulations of dark matter mixed in with gas and star formation recipes destined to model the formation of galaxies in a cosmological framework (e.g. Navarro et al. 1996, 1997). Although there are still a plethora of assumptions necessary to proceed from numerical simulations to observations which can test them, such an integrated approach seems to hold promising keys for future developments.