I have Platnium version. How do I get DARKRAI?!?! I KNOW the thingy is past but still, how do I capture him now without trading?
You would need an Action Replay or another cheat but I wouldn't reccomend using it, it can really mess up your game!
There currently isn't any way, you have to trade. Perhaps you'll get a chance to catch it in a future game, like they added tons of legendaries to heartgold/soulsilver.
You could have goten him through pokemon ranger shadows of almia, but it is too late now.