Dark data is the inactive or unused information your company keeps but doesn’t...

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

Dark data is the inactive or unused information your company keeps but doesn’t analyze—around 60-80% of enterprise data. It can increase storage and backup costs and raise breach risks, including ransomware exposure