Abstract: The discovery of high-energy cosmic neutrinos opened a new window of astroparticle physics. Revealing the sources is also relevant for solving the long-standing puzzle about the origin of cosmic rays. I will discuss theoretical implications of the latest results on high-energy neutrino observations, and demonstrate the power of multi-messenger approaches. In particular, I will highlight our recent developments about astrophysical neutrino emission from extragalactic gamma-ray dark sources and the Galactic plane. I will explore the potential of utilizing high-energy neutrinos as a probe of physics beyond the Standard Model.