Plenty of questions left unanswered

I just finished the anime and i really enjoyed it a lot, and i was expecting answers before it ended. Of course....i didn't get them.

1- Gantz: Why does it even exist? Who's the person inside the black ball?
2- What is all of this? A person dies and ends up there...HOW exactly?
3- The ending...what the hell? He doesn't return to that room, he's in the subway again. But exactly what he f is happening here?

I really ejoyed the show a lot. It showed how people can react to certain situations, which is awfully realistic and close to real life. It was damn good till the end, but i am so damn annoyed that there's no real explanation.