default-justification

default-justification is a variable defined in `fill.el'.
Its value is

left

• Automatically becomes buffer-local when set.
• This variable is safe as a file local variable if its value
  satisfies the predicate `symbolp'.

Documentation:
Method of justifying text not otherwise specified.
Possible values are `left', `right', `full', `center', or `none'.
The requested kind of justification is done whenever lines are filled.
The `justification' text-property can locally override this variable.

You can customize this variable.