Viral Dark Matter and Immune Calibration: Metagenomic Frontiers in Host–Virome Interactions

The human virome contains a large proportion of unclassified viral sequences collectively referred to as viral dark matter. These sequences reflect both genuine unexplored viral diversity and technical limitations arising from incomplete reference databases, fragmented assemblies, amplification bias, and insufficient functional annotation. This review evaluates how metagenomic workflows, including sampling, viral enrichment, sequencing, assembly, viral detection, and structure-based functional prediction, shape the identification and interpretation of viral dark matter. Virome-derived signals may influence host immunity through direct viral sensing and indirect phage-mediated modulation of bacterial communities. Although the connection between the virome and trained immunity remains emerging, current evidence permits a cautious immunological rheostat model in which persistent low-level viral exposure may contribute to innate immune calibration. Clinical associations between virome dysbiosis and inflammatory, metabolic, infectious, and mucosal disorders are considered with emphasis on distinguishing virome-specific effects from bacterial microbiome-driven mechanisms. Standardised pipelines, longitudinal sampling, functional validation, and integrated multi-omics approaches are needed to clarify the role of viral dark matter in human health and disease.