The early assembly of the most massive galaxies, and their contribution to the SFR density of the universe is currently one of the main challenges for galaxy formation models. Recently, a population of massive red sources already in place at z>2-3 have been discovered, and called HST-dark, because they are missed by optical/UV search but detected at MIR and often in the mm. Their characterisation remains uncertain, due to limited information available, and to different selection techniques. JWST is now allowing us to investigate the nature of HST-dark galaxies, and to reveal new populations of early massive galaxies. I will discuss the main properties of HST-dark galaxies, compared to those of JWST NIR-dark. For those ALMA detected, the molecular gas mass and depletion times show that they are likely on the way to quenching their SF. For ALMA HST-dark, there is a consensus on the presence of large amounts of dust, posing the problem of high-z dust production. To constrain dust properties and obscured SF at high-z we need to observe in the FIR. I will present the recently selected NASA PRobe far-Infrared Mission for Astrophysics (PRIMA), a FIR (25-235 μm) observatory operating in the 2030s in spectroscopy and imaging, that will allow us a leap forward in our knowledge of the high-z obscured universe.
Antonino Marasco