Matrix serves as the default theme for Gallery 2 and the base for several other Gallery themes. Gallery’s features are readily available through Matrix’s reliable cross-browser and cross-platform support. Matrix provides complete control of album pages by requiring site administrators to specify the number of columns and rows to show per page. Album and item options are grouped and presented in the sidebar located on the left-side of pages.
Center incomplete rows: http://galleryproject.org/node/33997