The first answer perfectly matches the reason that you gave: not knowing much about the matter.
The second answer matches the following reasons:
I don't have the right to say anything about this matter.
I am not involved in this matter.
I do not care about this matter.
I don't care what happens regarding this matter.
This matter is not important to me.
As you can see, some of the reasons may be good and some may be bad, depending on the situation. None of them mean "I don't know much about this matter," though.
So your first sentence is best.