We propose to utilize a unique multi-wavelength imaging and spectroscopic dataset of 15 massive lensing clusters, part of major Hubble programs, to perform new stringent tests on the Cold Dark Matter paradigm by comparing high-quality mass maps, dissected into the dark matter (DM) and baryonic components down to the innermost regions, with hydrodynamical simulations including different baryonic and DM micro-physics. Clusters’ mass distributions and density profiles are derived from kpc to Mpc scales, using all available mass probes: gravitational lensing, galaxy dynamics and Xray techniques. By constructing high-precision strong lensing models and magnification maps in the cluster cores, thanks to the combination of deep HST imaging and VLT integral field spectroscopy, we will exploit the magnification effect of these powerful gravitational lenses to probe the uncharted population of low-luminosity, low-mass (106-107 MSun) protogalaxies at z=3-7, zooming into star-forming complexes in the first two billion years, with an unparalleled resolution of 10-20 parsecs. By measuring their intrinsic physical properties and abundances, we will also determine their relative contribution to cosmic reionization and pave the way to future James Webb Space Telescope studies.