How can one change the default output alignment from left to center?
You can change the stylesheet of the notebook to style all output cells accordingly:
You can use
$PrePrint to add the option
TextAlignment -> Center to all output cells in the current session:
$PrePrint = Style[#, "Output", TextAlignment -> Center] &;
$PrePrint to the default value use
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.