  Multi-messenger high-energy astrophysics has currently achieved the potential
to unravel the origin of cosmic rays and how sources accelerate them, their
relation to the diffuse radiation in the extra-galactic space, and their role
to forge their galaxies of origin while they wander in their magnetic fields
for millions of years. Neutrino astronomy produced its major scientific
milestone with the discovery by IceCube of a diffuse flux at energies above
60~TeV with intensity comparable to a predicted upper limit to the flux from
extra-galactic sources of ultra-high energy cosmic rays. More recent results
provide the first strong evidence of a standalone neutrino source and a highly
probable coincidence of a neutrino alert with gamma rays. These results of
IceCube indicate that neutrino astronomy can complement photon astronomy also
providing insights into opaque sources of high-energy radiation. Starburst
galaxies and jetted black holes in active galaxies are favored candidates to
explain the diffuse cosmic neutrino background. Additionally, gamma-ray bursts
remain an intriguing mystery now enriched by joint observations of gamma-rays
and gravitational waves.
  The galactic diffuse flux, produced by cosmic ray interactions on the
interstellar matter of our galaxy and peaking at lower energies, is within the
reach of neutrino detectors. Together with the measured galactic gamma-ray flux
up to PeV energies, they will shed light on the knee region of cosmic rays and
the possible existence of dark matter in the Galactic plane. In the future,
more work will be done in IceCube and deep sea and lake neutrino telescopes to
use further low-energy cascades for cosmic source searches thanks to improved
descriptions of detection media and deep learning methods.
