tl,dr; We aren’t finding out what the black smoke monster is.
Actually that’s one of the questions I’m fairly sure we’ll get a sound answer to.
It’s actually one of the things that I’m worried about a “midi-chlorian-ish” explanation of, but I’m sure whatever they decide to give us is going to be good.
Things that will be left unanswered are probably things that Damon ad Carlton don’t have answers for and this is their excuse for not giving answers.
I’m fine with that. Lost has done a good job letting me “forget” about certain mysteries that might be a little too mysterious.