Dark data is the inactive information companies store but rarely use—often...

https://fast-wiki.win/index.php/How_Do_I_Stop_Dark_Data_From_Coming_Back_After_Cleanup%3F

Dark data is the inactive information companies store but rarely use—often making up 60-80% of their data. It matters because keeping all that unused data costs money, sometimes up to $2.5 million a year