[Brilliant Diamond & Shining Pearl] Rematch Diaga/Palkia?

I just got to the part where i challenge the eighth and final gym leader. And before that, I defeated Team Plasma and was going to face Dialga next...


Thing is... I accidentally defeated Dialga in battle and was assuming you'd be able to challenge it again later on so i can get another opportunity to catch it.

Please tell me this is true. Because it would be insanely stupid game design if you CAN'T battle it again.

you should be able to after you defeat the elite four.