(Marco Ajello, Mattia Di Mauro, Alberto Dominguez)
In a decade of sky-patrolling, the Fermi-Large Area Telescope (LAT) has detected significant gamma-ray emission from a variety of astrophysical objects. However, many more remain below the sensitivity limit of the LAT, and such cannot be constrained nor studied. Glaring examples are star-forming galaxies and extreme BL Lacs, which appear in any LAT catalog in very small numbers. Taking advantage of the lower detection threshold gained by the LAT in 10 years of observations, we have developed a novel stacking technique to detect and study these elusive source populations. In this talk, we will present the positive detection (beyond the LAT threshold) of star-forming galaxies and extreme BL Lacs and discuss their impact on the generation of the cosmic neutrino and extragalactic gamma-ray backgrounds.