What you are seeing are the disk buffers. This number (which appears as 'shared') represents storage that is being used to keep copies of information that has been recently read from disk, in anticipation that, should the same piece of data be needed again [a likely prospect...], a second disk-read will not be required.
You need not be alarmed by this; it is perfectly normal.
Under normal conditions, Linux will always use all of the memory that it has available, trying to put it to good use for something. If and when memory pressure develops, disk-buffers are among the first to go.
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