  Gamma rays reveal extreme, nonthermal conditions in the Universe. The Fermi
Gamma-ray Space Telescope has been exploring the gamma-ray sky for more than
four years, enabling a search for powerful transients like gamma-ray bursts,
solar flares, and flaring active galactic nuclei, as well as long-term studies
including pulsars, binary systems, supernova remnants, and searches for
predicted sources of gamma rays such as clusters of galaxies. Some results
include a stringent limit on Lorentz invariance violation derived from a
gamma-ray burst, unexpected gamma-ray variability from the Crab Nebula, a huge
gamma-ray structure in the direction of the center of our Galaxy, and strong
constraints on some Weakly Interacting Massive Particle (WIMP) models for dark
matter.
