The present accelerated expansion of the universe can be accounted for either by changing the matter sector (introducing some form of Dark Energy) or modifying gravity at large distances. I will discuss two ways to discriminate between the two options at short distances. The first relies on the gravitational effects from domain walls. While in GR domain walls produce a repulsive force, in infrared modifications of gravity such as the DGP model walls with small enough tension generate no gravitational field. In contrast with any other sources, this difference persists at arbitrarily small distances. I will also argue that the presence of Lorentz violating phenomena in high energy processes would indicate that gravity is modified at large distances. In IR modifications of gravity, the source of Lorentz violation can be screened and as a consequence may not affect the background spacetime. This leads to relax the bounds on Lorentz violating operators, to the point that their effects could be potentially observable in the near future.