Hey, I have found the solution.
What i have noticed that something is running on the background data, but I could not identify what app is sending so much data, so I started with backing up my whole phone.
I then started deleting all the unused apps. checking it the Android OS is still sending data during the course of the day. it was still using data. so i uninstalled facebook, facebook messenger and all social media apps, It started to decrease drastically!!!!!! but not to an acceptable usage.
I tehn deleted all my app ecept whatsapp. then it completely STOPPED, the Android OS background usage.
My suspicion is that an app was not functioning correctly and it kept running in the back ground. But unfortunately I can not pin point the precise app. My husband told me to Google/ Youtube Facebook messenger and background data. Be careful to have Facebook messenger on you phone, you give them authorization to make call,s and send ALL the data on you phone to FB, that why they know how to direct adverts to you when you are searching for specific products. Take time and google it. I was blown away. BUT newer the less, my background date have DECREASED to Approx 1mb pd. down from 100mb pd. I have in the meantime started reloading the apps i used, and it has not increased yet.
@Chanchan05, even though 500mb might not be a lot, I have Wifi at home so all my backups run from the home wifi, my real issue is the mobile date usage that i additionally have to pay for, in SA data is not cheap!!!!
GOOD Luck to everyone.
I hope this info will help.
Gwen