As background, lets start with what's already known to us: The Distortion/Reverse world is a parallel world reflecting the normal world where space and time behave abnormally, gravity changes massively with location, and all that good stuff. When spacetime gets damaged in the normal world, this place gets polluted. When stuff in the Reverse World gets damaged, explosions happen in the normal world. It's home to the Legendary Pokémon Giratina, the third part of the trio of Dialga (representing Time), and Palkia (representing space). Logically, what Giratina represents should be orthogonal to the two. Word of God says that Giratina and its world represent antimatter. What the fuck? What do antiparticles have to do with time and space?
As a brief aside, 106 years ago the mathematician Emmy Noether proved (in laymens terms) that every symmetry in how a physical system works implies a conservation law. For instance, if the physics of a system doesn't change with time (time invariance), then energy is conserved. symmetry under translation (physics works the same in a system no matter where one sets the origin. Ex:physics works the same if 3 meters to your left as it does where you are currently standing) implies conservation of [linear] momentum. Rotational symmetry (physics works the same in all directions) implies conservation of angular momentum. And so on. Now, as far as we can tell physics in the pokémon world obeys all these symmetries. However, it doesn't take much thought to notice that the conservation laws are broken routinely. Pokémon evolution draws energy/mass from nowhere. So do all sorts of attacks. Beam attacks like water gun or hyper beam that don't send the user backwards blatantly violate conservation of momentum. The mass from food that Pokémon eat vanishes entirely. What's going on here? Why does Pokémon seem to contradict Noether's theorem? You may be asking what this has to do with anything, or be about to reply 'Shadaki, you're looking way too much into this', but bare with me.
Back to the main topic: Giratina, its world, and antimatter. Now, while we're used to the idea of antimatter as simply being particles of opposite charge, that wasn't the original conception. When Paul Dirac was working on relativistic quantum mechanics, his Dirac Equation for spin 1/2 particles (i.e. electrons) allowed for negative energy solutions - which is a problem, as particles are assumed to have positive energy. His explanation for the negative solutions (and why positive energy particles don't decay into them) was that below the ground state is a ‘sea’ (yes, that Dirac Sea) of filled negative energy states (by Pauli's Exclusion Principle, only one spin 1/2 particle can exist in a specific state at a time). If the correct amount of energy is given, a particle from a negative state can be kicked up to a positive state, leaving a 'hole' in the sea. By all intents and purposes the 'hole' behaves like a particle with opposite charge. That is: Antimatter.
Headcanon: Giratina is more broadly the Dragon of Antimatter and Symmetries, and the '-' in 'space-time'. The Distortion/Reverse World is in turn a generalized version of the Dirac Sea as well as possibly serving as a way of mapping points in space to points in time. Anytime a Pokémon is doing something that seems to be violating a conservation law, its actually moving the 'violating' quantity to or from the Distortion/Reverse World. If one were to measure the space-time curvature of the otherworld in a specific location, one would find that it also changes depending on whether a Pokemon (or human?) is located in the corresponding sidereal location - except opposite from normal. Destroying something in the Reverse World causes its sidereal counterparts to try to fall to fill in energy state in the hole. The 'Pollution' comes from the world having trouble mapping points or genuine conservation law violations and figuratively raising its arms and saying 'fuck if I know, its supposed to be loosely like this or something'