  We report a realization of three-dimensional (3D) electromagnetic void space.
Despite occupying a finite volume of space, such a medium is optically
equivalent to an infinitesimal point where electromagnetic waves experience no
phase accumulation. The 3D void space is realized by constructing
all-dielectric 3D photonic crystals such that the effective permittivity and
permeability vanish simultaneously, forming a six-fold Dirac-like point with
Dirac-like linear dispersions at the center of the Brillouin Zone. We
demonstrate, both theoretically and experimentally, that such a 3D void space
exhibits unique properties and rich functionalities absent in any other
electromagnetic media, such as boundary-control transmission switching and 3D
perfect wave-steering mechanisms. Especially, contrary to the photonic "doping"
effect in its two-dimensional counterpart, the 3D void space exhibits an
amazing property of "impurity-immunity". Our work paves a road towards the
realization of 3D void space where electromagnetic waves can be manipulated in
unprecedented ways.
