Dream Rift

Dream Rifts were holes in the fabric of reality that were encountered by the J-Team mostly in Unova during the Unova-2 arc. These rifts were invisible to the naked eye, requiring special goggles to be observed, and spewed out various species of Dream World Pokémon. They were caused due to some mysterious scientists trying to capture Cresselia and managing to injure her, leading to the legendary losing control over the fabric of the Dream World.