  We examine the cosmological and astrophysical signatures of a "dark baryon,"
a neutral fermion that mixes with the neutron. As the mixing is through a
higher-dimensional operator at the quark level, production of the dark baryon
at high energies is enhanced so that its abundance in the early universe may be
significant. Treating its initial abundance as a free parameter, we derive new,
powerful limits on the properties of the dark baryon. Primordial
nucleosynthesis and the cosmic microwave background provide strong constraints
due to the inter-conversion of neutrons to dark baryons through their induced
transition dipole, and due to late decays of the dark baryon. Additionally,
neutrons in a neutron star could decay slowly to dark baryons, providing a
novel source of heat that is constrained by measurements of pulsar
temperatures. Taking all the constraints into account, we identify parameter
space where the dark baryon can be a viable dark matter candidate and discuss
promising avenues for probing it.
