Honestly I've heard reports of people who've died and have been brought back to life.
One interpretation I've heard was someone my mother knew, he died from a heart attack, he was later brought back to life. At the time he died, He said there was nothing but darkness, and he was wandering around in the dark. Makes you wonder if this is what "Ghosts" are.
Personally, I have several theories but I'm still completely unsure of what the afterlife is like. My best guess would be you go into some sort of afterworld, some like to call it Heaven.