Dark data is the inactive or unused information that makes up 55% of most...

https://domgodwinson38.gumroad.com/p/how-do-i-check-if-dark-data-contains-pii-or-phi

Dark data is the inactive or unused information that makes up 55% of most company data but still consumes storage and backup resources, costing millions annually. It also increases risk by providing a larger attack surface for ransomware or breaches