Yeah--it's..basically the embodiment of evil. The polar opposite to everything that the light stands for. It consumes people if they fall too deep into it.

[Like Riku had fallen.]

No, not really. The keyblade can destroy them and magic works on them to an extent. But I..a lot of my friends lost their worlds for awhile. We managed to get them back, seal away the darkness and then stop the people trying to release it again...

[But the damage was still done. There had been a lot of pain and a lot of suffering that went on and Sora wanted nothing more than to be able to fix it all.

To take it all away.]

Anywhere there's light, there's darkness. So the heartless can be anywhere they want to be.