It's a long story. Gold was here, Gold was dying like he did a while ago when I took the Darkness from him. I took it from him and spent six months with it. About four months into it, Killian disappeared. You weren't around then, but as it turns out the Dark One actually has a voice and it tries to convince you at every turn to do bad things. When Killian was around, the voice or ghost or whatever you want to call it was gone.
I went a little crazy with grief and tried to come up with a spell to bring him back. Blah blah the dimensions here, it backfired and I made a new Storybrooke and took Displaced people with me, giving them whole new lives and stories.