Then I dunno what is this. With the last one I meant that maybe you can't see some applications running in the task manager because you don't have rights, but I am not sure about this either.
The task manager always runs with admin rights when your account is admin. It doesn't ask you for elevation if UAC is in the default mode, but does ask if you set "always notify".
Sometimes processes allocate memory in such a way that it's hard to track which specific process owns it using the task manager. One possible clue you can get, if you enable the "Working Set (Memory)" column in the Details tab, and check that instead of private bytes, however, there can be much more that doesn't appear there.
By the way, if you're using Chrome, there may be memory leaks, especially with the latest updates. I've noticed that on my and other systems, certain tabs, most notably emails, cause repeated allocations, until it runs out of memory (if Chrome is 32-bit, otherwise it keeps allocating) - these reach more than 1GB. However, these are easy to track as you can press Shift+Esc in Chrome.
One popular program that might cause similar hard-to-track "leaks" is uTorrent. These go away once the process causing them is dead.
By the way, the screenshot you provided only displays memory occupied by a user, not by the whole system. The rest is very likely allocated by system apps or services that do not necessarily run on behalf of your user.
Discord, being a web application, can also eat a lot of memory (400MB and more) for no reason.
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