  A new mechanism has been proposed to simultaneously explain the presence of
dark matter and the matter-antimatter asymmetry in the universe. This scenario
predicts exotic $B$ meson decays into a baryon and a dark sector anti-baryon
($\psi_D$) with branching fractions accessible at $B$ factories. We present a
search for $B \rightarrow \Lambda \psi_D$ decays using data collected by the
$BABAR$ experiment at SLAC. This reaction is identified by fully reconstructing
the accompanying $B$ meson and requiring the presence of a single $\Lambda$
baryon in the remaining particles. No significant signal is observed, and
bounds on the $B \rightarrow \Lambda \psi_D$ branching fraction are derived in
the range $0.13 - 5.2\times 10^{-5}$ for $1.0 < m_{\psi_D} < 4.2$ GeV/$c^{2}$.
These results set strong constraints on the parameter space allowed by the
theory.
