I don't think antimatter was the original intent for Giratina, I think it was meant to be the god of the Spirit World.
- Turnback Cave being filled with Ghost-types
- Giratina's Pokdex entry in DP: "A Pokémon that is said to live in a world on the reverse side of ours. It appears in an ancient cemetery."
- The text in the room where Giratina is found reads: This is... That where life sparkles... That where life has faded... A place where two worlds overlap...
- One of the items you can find in Giratina's Room is a Reaper Cloth, which is called the Spirit World Cloth in Japanese, implying that it's from the Spirit World (the place where Dusknoir take lost spirits
- Even in Platinum, Cynthia makes a remark that Turnback Cave is said to "lead to the great beyond"
It being associated with antimatter seems to be a retcon made in Platinum to better fit in with Dialga/Palkia/s time-space powers
