documentclass{standalone}
usepackage{amsmath}
begin{document}

% NO ALIGNMENT
$begin{gathered}
|x|=
    begin{cases}
    amedspace{}text{if dots}\
    -amedspace{}text{if dots}
    end{cases}
end{gathered}$

% IMPROPER ALIGNMENT
$begin{gathered}
|x|=
    begin{cases}
    &amedspace{}text{if dots}\
    -&amedspace{}text{if dots}
    end{cases}
end{gathered}$

end{document}