Look up RETIRE on YouTube. There are videos he has made over glitches that can be used to obtain not just Darkrai, but also shaymin, infinite Arceuses, and Cresellia. (Don't know how to spell the last one.) I recommend that you do it yourself, it is also dangerous, so I recommend doing it on a save file you don't mind losing if you mess up bad. Though, you can transfer these pokemon up to other games if you like.