Dark data is all the information your company collects but never uses, often...

https://city-wiki.win/index.php/How_Do_You_Reduce_Backup_Costs_by_Shrinking_Primary_Storage%3F

Dark data is all the information your company collects but never uses, often hidden in files, emails, or backups. It can be 60-80% of your stored data, quietly taking up space and costing you around $2.5 million a year in wasted storage and management