Dark data is all the info your company collects but never uses—sometimes 60 to...

https://wiki-square.win/index.php/How_Do_I_Build_a_Business_Case_for_Dark_Data_Cleanup%3F

Dark data is all the info your company collects but never uses—sometimes 60 to 80% of it sits inactive. It matters because storing and managing this unused data can cost millions each year, often $2.5 million or more