  Because cosmic rays are charged particles scrambled by magnetic fields,
combining direct measurements with other observations is crucial to
understanding their origin and propagation. As energetic particles traverse
matter and electromagnetic fields, they leave marks in the form of neutral
interaction products. Among those, gamma rays trace interactions of nuclei that
inelastically collide with interstellar gas, as well as of leptons that undergo
Bremsstrahlung and inverse-Compton scattering. Data collected by the Fermi
Large Area Telescope (LAT) are therefore telling us the story of cosmic rays
along their journey from sources through their home galaxies. Supernova
remnants emerge as a notable gamma-ray source population, and older remnants
interacting with interstellar matter finally show strong evidence of the
presence of accelerated nuclei. Yet the maximum energy attained by shock
accelerators is poorly constrained. Cygnus X, a massive star-forming region
established by the LAT as housing cosmic-ray sources, provides a test case to
study the impact of wind-driven turbulence on the early propagation.
Interstellar emission resulting from the large-scale propagation of cosmic rays
in the Milky Way is revealed in unprecedented detail that challenges some of
the simple assumptions used for the modeling. Moreover, the cosmic-ray induced
gamma-ray luminosities of galaxies scale quasi-linearly with their massive-
star formation rates, and suggests that for most systems a substantial fraction
of energy in cosmic rays escapes into the intergalactic medium. The nuclear
production models and the distribution of target gas and radiation fields, not
determined precisely enough yet, are key to exploiting the full potential of
gamma-ray data. Nevertheless, data being collected by Fermi and complementary
observations are bringing us ever closer to solving the cosmic-ray mystery.
(abridged)
