Implications of the intriguing constant inner mass surface density observed in dark matter halos

It is known for long that the observed mass surface density of cored dark matter (DM) halos is approximately constant, independently of the galaxy mass (i.e., rhoc X rc simeq constant}, with rhoc and rc the central volume density and the radius of the core, respectively). Here we review the evidence supporting this empirical fact as well as its theoretical interpretation. It seems to be an emergent law resulting from the concentration-halo mass relation predicted by the current cosmological model, where the DM is made of collisionless cold DM particles (CDM). We argue that the prediction rhoc X rc simeq constant is not specific to this particular model of DM but holds for any other DM model (e.g., self-interacting) or process (e.g., stellar or AGN feedback) that redistributes the DM within halos conserving its CDM mass. In addition, the fact that rhoc X rc simeq constant is shown to allow the estimate of the core DM mass and baryon fraction from stellar photometry alone, particularly useful when the observationally-expensive conventional spectroscopic techniques are unfeasible.