Here are some ways to obtain Darkrai in the past:
* Event Distribution: Darkrai was distributed via special events in the past, usually at GameStop stores or via Nintendo Wi-Fi Connection.
* Mystery Gift: You might have been able to receive Darkrai via Mystery Gift, but these opportunities are often limited and time-sensitive.
Currently, there's no way to obtain Darkrai in Pokémon Platinum without resorting to trade with someone who previously obtained it through a special event.
Let me know if you have any other Pokémon questions!