Cannot find the process using most of RAM

I have 8 GB RAM running on Ubuntu. I notice that a lot of memory is used even when I don't run anything programs. I used top command to view the processes sorted by memory they used. It showed that about 5GB was being used while the sum of memory used by processes are about < 20% of 8GB, which is about 1-2 GB.

Is there any possibility that some process using a lot of RAM is invisible from any program like top?

For now, superuser does not allow me to post image because I am a beginner. Please click the following link to see info of my computer's RAM.

Click here to see the top command's result

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