Probing the galactic center by gravitational lensing
  The nature of Galactic Center could be probed by lensing experiments capable of testing the spatial and velocity distributions of stars nearby and beyond it. Several hypotheses are possible (e.g. massive neutrino condensation, boson star) which avoid the shortcomings of the supermassive black hole model. 
S.Capozziello, G.Iovane, G.Lambiase, D.F.Torres