How do you catch Darkrai without cheats?

In platinum, no I don't think you can...

but in diamond and pearl you either need an event (for example: go to toys-r-us to get manaphy) to get darkrai or, send it to a friend using Pokemon ranger:shadows of almia

then have them link trade it to you.

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note:a darkrai event might come up soon (for platinum), i dont really know