Dark Matter in Galaxies: evidences and challenges
  The evidence of the phenomenon for which, in galaxies, the gravitating mass
is distributed differently than the luminous mass, increases as new data become
available. Furthermore, this discrepancy is well structured and it depends on
the magnitude and the compactness of the galaxy and on the radius, in units of
its luminous size $R_{opt}$, where the measurement is performed. For the disk
systems with $-13\geq M_I\geq -24$ all this leads to an amazing scenario,
revealed by the investigation of individual and coadded rotation curves,
according to which, the circular velocity follows, from their centers out to
their virial radii, an universal profile $V_{URC} (r/R_{opt}, M_I)$ function
only of the properties of the luminous mass component. Moreover, from the
Universal Rotation Curve, so as from many individual high-quality RCs, we
discover that, in the innermost regions of galaxies, the DM halo density
profiles are very shallow. Finally, the disk mass, the central halo density and
its core radius, come out all related to each other and to two properties of
the distribution of light in galaxies: the luminosity and the compactness. This
phenomenology, being absent in the simplest $\Lambda CDM$ Cosmology scenario,
poses serious challenges to the latter or, alternatively, it requires a
substantial and tuned involvement of baryons in the formation of the galactic
halos. On the other side, the URC helps to explain the two-accelerations
relationship found by McGaugh et al 2016, in terms of only well known
astrophysical processes, acting in a standard DM halos + luminous disks
scenario
