Dear David and Gustavo.
Please forgive me for my absent-mindedness.
At first, I was right when I thought that ''the matter'' was a subject complement.
But then my mind appeared the sentence " What is your hobby?"
Comparing the two sentences: ''What is the matter?''; ''What is your hobby?'', I was confused, they are so identical in grammatical structure.
Now I understand completely. I promise I won't ask such a kind of question.
Thank you both for helping me with all your heart and I am a student with my bad mind which worth punishing.