Dark data is the untapped information stored but not actively used—often making...

https://blogfreely.net/samuel-wang09/how-do-i-check-if-dark-data-contains-pii-or-phi

Dark data is the untapped information stored but not actively used—often making up 55% of enterprise data. It drives up storage and backup costs and increases breach risk since unmanaged data is more vulnerable to ransomware