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

https://mill-wiki.win/index.php/S3_Intelligent_Tiering_for_Inactive_Data_%E2%80%93_What_Should_I_Watch_For%3F

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