  The H.E.S.S. survey of the inner Galaxy in the very-high-energy gamma-ray
domain has led to the discovery of many extended sources, some of which do not
appear to be associated with any obvious counterpart at traditional wavelengths
(radio, infrared and X-ray). In this contribution, preliminary H.E.S.S. results
on one of these so-called "dark" sources, namely HESS J1503-582, are presented.
After introducing the properties of this source candidate, results of the
search for counterparts in several astronomical windows are shown. Finally, its
possible association with a "Forbidden-Velocity-Wing", a characteristic 21 cm
HI line structure that appears as a deviation from the canonical Galactic
rotation curve, is discussed.
