In spite of the many successes of the ΛCDM cosmology on large scales, there are some fundamental difficulties on small, galactic scales. One such difficulty is the distribution and quantity of dark matter in galaxies. A central assumption of the current ΛCDM cosmological model is that galaxies form and evolve inside extended dark matter halos (e.g. White & Rees, 1978; Davis et al., 1985). These halos are possibly universal in their density profile with steep inner density profiles forming a central cusp (e.g. Navarro et al., 1997; Merritt et al., 2005). found ρ ∝ r−1.4 using higher

Extended dark matter halos have been detected observationally using various techniques, predominantly through dynamical tracers in spiral galaxies (e.g. van Albada et al., 1985), early-type galaxies (e.g. Loewenstein & White, 1999; Gerhard et al., 2001), and clusters of galaxies (e.g. Zwicky, 1937b), but also through other techniques such as combined weak and strong gravitational lensing (e.g. Kneib et al., 2003). All these studies lead to the conclusion that dark matter halos are extended and have steep and divergent (i.e. cuspy) inner density profiles (with ρ ∝ r−1 or steeper). Large dark matter cores and very low dark matter central densities, seem to be excluded.