First mentioned by Old Kai in Dragon Ball Z, whenever a wish is made using the Dragon Balls, an equal amount of negative energy is created alongside the beneficial wish-granting energy. To prevent this energy from causing problems, the Dragon Balls absorb this dark energy. The dark energy stored in the Dragon Balls disperses only after 100 years, and the Dragon Balls can only store a limited amount of it. After used to make a wish, the Dragon Balls scatter across the planet, giving them time to dispel the dark energy harmlessly (since it is expected that it would take years for someone to find all the Dragon Balls and summon Shenron again, especially since most people do not know about their power). Unfortunately, this countermeasure was proved near useless when Bulma invented the Dragon Radar, allowing the Z Fighters to easily find the Dragon Balls and summon Shenron many times. Eventually, during Dragon Ball GT, all seven Dragon Balls were filled to full capacity, causing them to crack. When these damaged Dragon Balls were used to summon Shenron to restore the Earth after Goku's battle with Super 17, the Dragon Balls released years worth of dark energy instead, which manifested into an evil Black Smoke Shenron, who in turn swallowed the Dragon Balls and split into seven Shadow Dragons.
Despite the fact that the seven Shadow Dragons were all created by individual wishes, not every wish made spawned a Shadow Dragon. For example, none of the wishes made after the Cell Games to revive Cell's victims and remove the bombs from the Androids spawned a Shadow Dragon. It is unknown what became of the negative energy of those wishes.