Dark data is the inactive or unused information that piles up in your storage,...

https://mike-wiki.win/index.php/Pure_Storage_FlashBlade_and_Unstructured_Data_Growth_-_What_to_Do

Dark data is the inactive or unused information that piles up in your storage, often making up 60-80% of what you keep. Holding on to this data wastes money on storage and backup, sometimes costing millions annually