  The plateau at 1/3 of the saturation magnetization, $M_s$, in the metamagnet
CeSb is accompanied by a state of ferromagnetic layers of spins in an
up-up-down sequence. We measured $M$ and the specific heat, $C$, in the
plateau, spin wave analyses of which reveal two distinct branches of
excitations. Those with $\Delta S_z = 1$ as measured by $M$, coexist with a
much larger population of $\Delta S_z = 0$ excitations measured by $C$ but
invisible to $M$. The large density of $\Delta S_z = 0$ excitations, their
energy gap, and their seeming lack of interaction with $\Delta S_z = 1$
excitations suggest an analogy with astrophysical dark matter. Additionally, in
the middle of the plateau three sharp jumps in $M(H)$ are seen, the size of
which, $0.15 $%$M_s$, is consistent with fractional quantization of
magnetization-per-site in the down-spin layers.
