Blank areas between thumbnails

I think the next step would be to delete the thumbnail cache.  I assume that it is the same on Mac as Windows.  So look for the thumb.5.cache file in your catalog folder and delete it.  Depending on the size of your catalog, it may take a while to rebuild the thumbnails.  Before deleting the file, you may want to back it up or just rename it.  The file should then be rebuilt automatically when you relaunch the Organizer.

I have never seen or heard of anything like your screenshots.   What happens if you use full screen view of an image - do photos scroll normally one at a time, without any gaps showing? Have you, in fact, deleted large numbers of files from the catalog?

The only suggestion I can think of right now is to repair and optimize the catalog from the File>Manage Catalogs dialog.

PS I know almost nothing about Macs.

It looks like you are in the 'detailed' view, which is the only one able to display date_taken, file name and the various stacks/version sets icons.

You seem to want the 'adaptative view', which only shows the thumbnails.

You switch from one mode to antother with the shortcut 'Ctrl D'.

(I only use the detailed view).