How Did the IGM Become Enriched?
  The enrichment of the intergalactic medium with heavy elements is a process that lies at the nexus of poorly-understood aspects of physical cosmology. We review current understanding of the processes that may remove metals from galaxies, the basic predictions of these models, the key observational constraints on enrichment, and how intergalactic enrichment may be used to test cosmological simulations. 
Anthony Aguirre, Joop Schaye