(thin space) should do
Note that does not have the same width as an — (—); to separate numbers you should use a narrow no-break space (U+202F).
As others have mentioned, you are better off using the CSS property word-spacing to define the width of your spaces. It is probably a good idea to combine it with white-space:nowrap; to avoid breaking lines between groups of numbers and keep the full number on a single line.