Possibly malware. Download malwarebytes and run a full system scan. it could also be excess programs on your computer that are consuming resources in the background, or maybe windows is doing stuff. Remove programs and apps you don't use and go to task manager and go to startup then disable anything you don't want automatically running when you turn on your pc.
If the total does not add up, go to Resource Monitor instead of Task Manager as it shows hidden processes as well; in the memory tab, click on the header of the Working Set column to sort the processes by their RAM usage and see what's using your RAM
task manager only shows your users usage as well. not system
Download Process explorer and run it as admin (it comes from Microsoft so its safe)
the default view is tree structure meaning like your task manager screen, it will show what processes are under each service, but unlike task manager, it shows the ram usage of each part so you can see what is eating your ram
Private bytes = Ram + page file usage
Working set = actual ram usage
This page shows what all the colours and headings mean, link at bottom of it shows how to use it to find problems. You can right click headers and run an av scan from within the program.