  To comprehend the shadow of a black hole in a general spacetime, we have
investigated the concept of the maximal black room (MBR). The boundary of the
MBR is a non-spacelike hypersurface that contains at least one null geodesic
tangent to its boundary, which we refer to as the rays' surface. Our aim is to
explore the geometry of this surface.
  From our current study, we have observed that the boundary of the MBR is
globally unstable. Leveraging this instability, we can determine the boundary
based on the physical choice of the initial spacelike hypersurface and the
orthogonal condition at that point.
  Upon examining the existence of the MBR, we can observe that it encompasses
any arbitrary black hole. Additionally, we can investigate the possibility of
using the MBR to differentiate a black hole from an exotic star with a photon
sphere. In spherically symmetric spacetime, subject to energy conditions, the
rays' surface encloses a black hole, a naked singularity, or exotic matter that
has an inner universe.
