The evidence for unusual gravity from the large-scale structure of the   Universe
  Under the assumption that General Relativity (GR) correctly describes the phenomenology of our Universe, astronomical observations provide compelling evidence that (1) the dynamics of cosmic structure is dominated by dark matter (DM), an exotic matter mostly made of hypothetical elementary particles, and (2) the expansion of the Universe is currently accelerating because of the presence of a positive cosmological constant Lambda. The DM particles have not yet been detected and there is no theoretical justification for the tiny positive Lambda implied by observations. Therefore, over the last decade, the search for extended or alternative theories of gravity has flourished. 
A. Diaferio