  We examine under what circumstances the INTEGRAL/SPI 511 keV signal can
originate from decays of MeV-scale composite states produced by: (A)
thermonuclear (type Ia) or (B) core collapse supernovae (SNe). The requisite
dynamical properties that would account for the observed data are quite
distinct, for cases (A) and (B). We determine these requirements in simple
hidden valley models, where the escape fraction problem is naturally addressed,
due to the long lifetime of the new composite states. A novel feature of
scenario (A) is that the dynamics of type Ia SNe, standard candles for
cosmological measurements, might be affected by our mechanism. In case (A), the
mass of the state mediating between the hidden sector and the SM $\ee$ could be
a few hundred GeV and within the reach of a 500 GeV $\ee$ linear collider. We
also note that kinetic mixing of the photon with a light vector state may
provide an interesting alternate mediation mechanism in this case. Scenarios
based on case (B) are challenged by the need for a mechanism to transport some
of the produced positrons toward the Galactic bulge, due to the inferred
distribution of core collapse sources. The mass of the mediator in case (B) is
typically hundreds of TeV, leading to long-lived particles that could, under
certain circumstances, include a viable dark matter candidate. The appearance
of long-lived particles in typical models leads to cosmological constraints and
we address how a consistent cosmic history may be achieved.
