How to automatically center the alignment of all output cells?

You can use $PrePrint to add the option TextAlignment -> Center to all output cells in the current session:

$PrePrint = Style[#, "Output", TextAlignment -> Center] &;

To restore $PrePrint to the default value use

$PrePrint =.

If the notebook contains output cells created before $PrePrint is invoked, you can use

Scan[SetOptions[#, TextAlignment -> Center] &, Cells[CellStyle -> "Output"]]

to change their alignment.

You can change the stylesheet of the notebook to style all output cells accordingly:

You can find several questions on how to permanently install such stylesheets or how to programmatically change it in the official documentation and on this site, for example here and here.