Dark matter dominates the dynamics of cosmic structures.
Quantifying the relative distribution of dark and luminous
matter is a major challenge of modern cosmology.
I will present the caustic technique, a method which uses the
position and redshift of galaxies to measure the mass in the outer
regions of clusters; here traditional methods, based
on the assumption of dynamical equilibrium, do not hold.
The mass-to-light ratio measured in the outskirts of
the CAIRNS clusters provides relevant constraints
on the universe matter density and on galaxy formation models.