  The mean iron abundance observed in the intergalactic medium (IGM) within
galaxy clusters and without galaxy clusters is consistent with the mean amount
of iron per unit volume in the Universe which has been produced by standard
supernova (SN) explosions with a rate proportional to the cosmic star-formation
rate. If most SNe took place inside galaxies, then the IGM could have been
enriched with their metals by galactic winds and jets that swept most of the
galactic gas with the SNe ejecta into the IGM. A significant fraction of the
early SNe, however, could have taken place outside galaxies or within dwarf
galaxies, which were later disrupted by tidal interactions, and/or mass loss
through fast winds, SN ejecta and jets. Little is known about such
intergalactic SNe at high red-shifts. They could have occurred primarily in
highly obscured environments, avoiding detection. Supporting evidence for
intergalactic SNe is provided by SNe associated with gamma ray bursts (GRBs)
without a host galaxy and from the ratio of well localized GRBs with and
without a host galaxy. A direct test of whether a significant contribution to
the iron abundance in the IGM came from intergalactic SNe would require the
measurement of their rate per comoving unit volume as function of red-shift.
This may be feasible with IR telescopes, such as the Spitzer Space Telescope.
