  We provide a thorough study of a carbon divacancy, a fundamental but almost
unexplored point defect in graphene. Low temperature scanning tunneling
microscopy (STM) imaging of irradiated graphene on different substrates enabled
us to identify a common two-fold symmetry point defect. Our first principles
calculations reveal that the structure of this type of defect accommodates two
adjacent missing atoms in a rearranged atomic network formed by two pentagons
and one octagon, with no dangling bonds. Scanning tunneling spectroscopy (STS)
measurements on divacancies generated in nearly ideal graphene show an
electronic spectrum dominated by an empty-states resonance, which is ascribed
to a spin-degenerated nearly flat band of $\pi$-electron nature. While the
calculated electronic structure rules out the formation of a magnetic moment
around the divacancy, the generation of an electronic resonance near the Fermi
level, reveals divacancies as key point defects for tuning electron transport
properties in graphene systems.
