What is the difference between "吗 (mǎ)" and "么 (ma)"?
They are different in tone side, for example when you ask some one if have dinner, you can use 你吃晚饭了吗? or 你吃晚饭了么? Both are OK, because it's not strong sense of doubt, just ask, not a question style.

But you can not use 么 in 你拿得出证据证明他的清白吗? In the sentence "你拿得出证据证明他的清白吗?" has a style of you have to give me the answer, and mostly you can not. 

So, mostly, you can use 吗 and 么,but with the strong tone, especially rhetorical question you can just use 吗.