  A large part of the Galactic sources emitting very high energy (VHE; >
10^{11} eV) gamma-rays are currently still unidentified. The evolution of
Pulsar Wind Nebulae (PWNe) plays a crucial role in interpreting these sources.
The time-dependent modeling of PWNe has been tested on a sample of well-known
young and intermediate age PWNe; and it is currently applied to the full-sample
of unidentified VHE Galactic sources. The consequences of this interpretation
go far beyond the interpretation of "dark sources" (i.e. VHE gamma-ray sources
without lower energies, radio or X-ray, counterparts): e.g. there could be
strong implication in the origin of cosmic rays and (when considering a
leptonic origin of the gamma-ray signal) they can be important for
reinterpreting the detection of starburst galaxies in the TeV gamma-ray band.
Moreover, the number of Galactic VHE sources is currently increasing with
further observation by Imaging Atmospheric Cherenkov Telescopes (IACTs) and by
the advent of more sensitive water Cherenkov telescopes such as HAWC
(High-Altitude Water Cherenkov Observatory); therefore the physical
interpretation of unidentified sources becomes more and more crucial.
