Since "無" is usually used in conjunction with another word, Eg. 無知/博識 むち/はくしき (ignorance-knowledgeable) or 無理/道理 むり/どうり (unreasonable-reasonable) I don't think it has an exact opposite on its own. If you are trying to get the meaning un, none or without, maybe you should use nashi (なし) which uses the same kanji with the kunyomi 無し(Japanese pronounciation)
I am afraid my explaination may not be too good but it would be like using the "un" in unreal to indicate nothing or zero. It lacks a real meaning so therefore it has not got a perfect opposite.
I hope this helps and it is a great question that got me thinking about all the subtle differences between Japanese and all other languages.
Cheers,聡
