  The near future mm/sub-mm VLBI experiments are ambitious projects aiming at
imaging the "shadow" of the supermassive black hole candidate at the center of
the Milky Way and of the ones in nearby galaxies. An accurate observation of
the shape of the shadow can potentially test the nature of these objects and
verify if they are Kerr black holes, as predicted by general relativity.
However, previous work on the subject has shown that the shadows produced in
other spacetimes are very similar to the one of the Kerr background, suggesting
that tests of strong gravity are not really possible with these facilities in
the near future. In this work, I instead point out that it will be relatively
easy to distinguish black holes from wormholes, topologically non-trivial
structures of the spacetime that might have been formed in the early Universe
and might connect our Universe with other universes.
