Hi,
You do not have memory to not use it. Linux will take all memory it can and will reuse/free it if needed.
You are 50% empty a lot of room.
I see that on the top kwin_x11 is using 4KM → 4GB.
Note that you have VIRT (virtual memory including everything) and RES (Resident set size which is basically the real memory allocated for this process only).
You can also see than you are not using swap (Almost) which means your machine is not and was not in need of memory (RAM).
https://serverfault.com/questions/138427/what-does-virtual-memory-size-in-top-mean
To check your memory use the command free :
[Manjaro ~]# free
total used free shared buff/cache available
Mem: 7827 884 2932 208 4010 6230
Swap: 3999 61 3938
You can see the buff/cache there. This is memory used to store volatile data that stay there to avoid disk access like cache, libs etc …
You can clean the cache to think you gain memory but you will loose in peformance as you’ll need to access the disk again instead of taking advantage of your ram.
See : How to Clear RAM Memory Cache, Buffer and Swap Space on Linux
If I do that on my laptop I will have more free mem but I have gained nothing but lost my caches.
[Manjaro ~]# sync; echo 3 > /proc/sys/vm/drop_caches
[Manjaro ~]# free
total used free shared buff/cache available
Mem: 7827 2003 5026 221 798 5231
Swap: 3999 324 3675
[Manjaro ~]#
More info : Free Command Output Explanation In Linux/Unix - LookLinux
Last thing memory usage of processes are stored in /proc there is an easy to use code to display that (more readable than top) :
I am pretty sure there is nothing to worry about, use you ram and have fun.