I love the lore surrounding Legendaries; it's literally my favourite part of Pokemon. But some of it's a bit weird. Like Giratina's Lore.
So here's the myth, backed up by theory from me and the Youtube channel TheNationalDex (check 'em out, they do weekly Pokemon trivia and battle strategy, it's amazing):

In the beginning, we had the Egg, which gave birth to Arceus ("The Original One"). From itself, Arceus created Dialga and Palkia, and the universe was born out of their respective dominion over time and space. MATTER was born. That's all there is to it, right?
WRONG.
Einstein once quoted, "for every action there is an equal and opposite reaction". Broken down, this means there can be no Yin without a Yang. Nothing can exist without an opposite. Einstein created a revised form of his famous formula "E=mc-squared" in order to further explore his theory. When solved it yielded two solutions: a positive and a negative.
What is everything made up of? Matter. The positive.
What is the opposite of matter? Antimatter. The negative.

Making sense now?

Dialga and Palkia essentially created matter. Going back to Einstein, matter, like anything, can't exist without an opposite - antimatter. THEREFORE - could Giratina have been created alongside Dialga and Palkia to provide an opposite to the matter they created? Was it born out of necessity? Junichi Masuda has stated that Giratina is the personification of antimatter - a force in direct opposition to time and space. Remember how nothing can exist without an opposite? Maybe there would be no time or space had Giratina not come into existence!

If I haven't confused the shit out of you by now - haha - here's some more. Dumbed down a large amount, if a particle of matter meets a particle of antimatter they will expel an incredible amount of energy and annihilate each other. In other words, incredible destruction on a microscopic level. This is important as it may explain why Giratina was banished to the Distortion World. Remember how its Platinum Pokedex entry states "it was banished for its violence"? Well, Giratina is antimatter incarnate. The world Arceus created was, of course, made up of matter. Basic science. And what happens when matter and antimatter meet? COMPLETE AND UTTER CELLULAR ANNIHILATION. Giratina merely EXISTING was tearing the world to pieces. So maybe Arceus, with its godlike powers, opened a portal to another plane and banished Giratina there, saving the world. Yay.
HOWEVER.
Giratina's cellular destruction began to annihilate all matter in the alternate dimension too - until there was none. All that was left was a land made entirely of antimatter. No rules are followed. Time doesn't flow. Space isn't stable. Everything is in opposition. The Distortion World. Much explanation. Such theory. Wow.

But there's still a couple of issues.
1). In Platinum (and the movie "Giratina and the Sky Warrior"), HUMANS visit the Distorton World. Humans are made of matter. So if they entered a space made entirely of antimatter, wouldn't they die? Like, immediately? Remember that antimatter and matter annihilate one another. I have no idea how to address this issue, so tell me what you think.
2). Giratina has appeared in Arceus's world a few times throughout the games and anime, so why didn't everything go to shit again? I CAN answer this one. My theory is that Giratina's altered forme (the forme it can only take in Arceus's world) is kind of like a container, possibly created by Arceus, to stop Giratina's power from going out of control and causing annihilation - kind of like Golurk's seal. This explains the altered forme's more defensive stat spread compared to Origin forme, which is more attack based. This could also explain why altered forme resembles a ground-based insect while Origin forme is more flight-oriented. Some insects mature to adulthood by having their winged, adult forms EMERGING from the husk of their ground-based nymph forms. Kinda similar to Giratina's formes, eh? This is also similar to how Origin forme is more offensive than altered, as an adult insect is generally more of a predator than the younger nymph.
So maybe the altered form is like a power-restricting cocoon which can only be shed in the Distortion World, releasing Giratina's full power!

Sorry if I've just melted your brains - this stuff confuses me too. Keep in mind that this is 90% theory, 4% Pokemon mythology and 6% science, so it may not all be true.
Once again, this was put together by my own theories and those of TheNationalDex. Seriously, check the channel out. They rock.

As always, leave some thoughts of your own!