Abstract. The riddle of the origin of Cosmic Rays is open since more than one century. Gamma ray observations above 100 MeV reveal the sites of cosmic ray acceleration and recently we got the experimental proof of hadronic acceleration in Supernovae Remnants, however new questions rised and no final answer has been provided so far. In the last years the knowledge in this field of research widely increased, however almost 50% of the TeV (> 10^{12} eV) Galactic sources are still unidentified; at GeV (> 10^9 eV) energies, 67% of EGRET sources were unidentified and also with the newer generation of gamma-ray satellites we have the same result: in fact, at low Galactic latitudes (b <10 deg), 62% of the Fermi LAT detected sources have no formal counterpart. Hence understanding the high energy unidentified sources will be a crucial brick in solving the whole riddle of Cosmic Rays origin. Several examples will be shown, underlining the importance of the so-called "dark sources". Both theoretical aspects (with particular emphasis to the so-called Ancient Pulsar Wind Nebulae scenario) and their observational proofs will be discussed.