Dark data is the inactive or unused information sitting on your storage, often...

https://reidyxab469.iamarrows.com/do-i-have-to-move-petabytes-before-i-know-what-i-have

Dark data is the inactive or unused information sitting on your storage, often making up 60-80% of your files. It matters because this data can expose your business to ransomware attacks and data breaches while driving up storage and backup costs