  It has been proposed recently that a previously unobserved neutron decay
branch to a dark matter particle ($\chi$) could account for the discrepancy in
the neutron lifetime observed in experiments that use two different measurement
techniques. One of the possible final states discussed includes a single $\chi$
along with an $e^{+}e^{-}$ pair. We use data from the UCNA (Ultracold Neutron
Asymmetry) experiment to set limits on this decay channel. Coincident
electron-like events are detected with $\sim 4\pi$ acceptance using a pair of
detectors that observe a volume of stored Ultracold Neutrons (UCNs). The summed
kinetic energy ($E_{e^{+}e^{-}}$) from such events is used to set limits, as a
function of the $\chi$ mass, on the branching fraction for this decay channel.
For $\chi$ masses consistent with resolving the neutron lifetime discrepancy,
we exclude this as the dominant dark matter decay channel at $\gg~5\sigma$
level for $100~\text{keV} < E_{e^{+}e^{-}} < 644~\text{keV}$. If the
$\chi+e^{+}e^{-}$ final state is not the only one, we set limits on its
branching fraction of $< 10^{-4}$ for the above $E_{e^{+}e^{-}}$ range at $>
90\%$ confidence level.
