Explanation

Dark phase of photosynthesis or Calvin cycle takes place in stroma of chloroplast. First step of dark phase is Carboxylation. In Carboxylation, carbon dioxide is combined with RuBP to form first stable compound. $6RuBP + 6CO_{2}\overset{RuBP\ carboxylase}{\rightarrow}12\ phosphoglyceric\ acid$ $NADP^{+}$acts as hydrogen acceptor.