# Comment an equation without creating extra space

I try to make a comment on the right (or left) of one line of proof. But the align* environment automatically move the whole equations to the left to leave space for the comment. But I still want the equations to be in the center (the comment is short enough that it won't be conflicting the equations). How could I achieve this?

To be more specific, I used the code

 \begin{align*}
equation0 &= equation1
&= equation2
&= equation3 && \text{make a comment here}
&= equation4
\end{align*}


I just want to put \text{make a comment here} on the right without affecting the original alignment of the equations.

-

Omit the && and use \rlap{\text{make a comment here}}, which causes the comment to have no width "to the right", meaning it starts at the same place but doesn't actually take up any space in the alignment. For a comment on the left, use \llap instead of \rlap, which causes leftward space to be ignored.
In response to the question in the comment: in align* or equation* specifically (or any other unnumbered environment) you can hijack the "equation label" to contain a comment by using \tag*{\text{make a comment here}} rather than \rlap. This will place the comment on the left if amsmath was loaded with the leqno option. In a numbered equation, it is not a good idea to put something flush right.
Instead of \rlap{...} I would recommend \makebox[0pt][l]{...} that's slightly less efficient but also less surprising in other contexts (try starting a paragraph with \rlap). – egreg Apr 14 '13 at 8:37