  We study the effect of a dibaryon, S, in the mass range 1860 MeV < m_S < 2054
MeV, which is heavy enough not to disturb the stability of nuclei and light
enough to possibly be cosmologically metastable. Such a deeply bound state can
act as a baryon sink in regions of high baryon density and temperature. We find
that the ambient conditions encountered inside a newly born neutron star are
likely to sustain a sufficient population of hyperons to ensure that a
population of S dibaryons can equilibrate in less than a few seconds. This
would be catastrophic for the stability of neutron stars and the observation of
neutrino emission from the proto-neutron star of Supernova 1987A over ~ O(10)s.
A deeply bound dibaryon is therefore incompatible with the observed supernova
explosion, unless the cross section for S production is severely suppressed.
