The Most Important Question in the Metaphysics

I wrote the following to a friend. To me it seems perhaps the most important question in the Metaphysics:

It seems that somewhere in the Met, Aristotle should give a justification for the possibility of the existence of non-material substances. If in Lambda he is going to assert that such substances exist, then he needs to give an argument, it seems, that such substances *can* exist. But do you think that anywhere in the central books he is arguing with that end in view--that is, that he is setting down ideas, or considerations, which imply that there is no obstacle to forms existing apart from matter?

I suppose the alternative is simply to hold that when, in Lambda, he argues that something must exist which has no potentiality, then ipso facto he shows that it has no matter. But, still, one wants to know how such a thing could exist, if in every other case, or nearly so, forms exist through matter.

My friend replied that Z seemed to do no more than leave open the possibility of immaterial substances, but that perhaps Q 8 was the place to look for something more definite. My friend also suggested that an 'immaterial susbstance' should perhaps not be understood as a 'form existing without matter.' (But then how should it be understood?)

Do you have an idea how to answer my question?