Part of the reason it is never fully explained is that it can't be. Tied in with the "horrors beyond imagining" sense you get from the Cthulhu mythos, it would take away some of the mystique to have everything perfectly detailed and laid out. Mostly, people just don't know. That's where the fear comes from; we fear that which we don't understand.
For a more direct answer to your question, most of what is known about the Dark Tapestry can be found in the Pathfinder Wiki entry.
Creative Director
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The "Dark Tapestry" is basically "The dark places between the stars." It's a more fantasy-feeling version of saying "deep space," basically. We don't have a single place where we really talk much about the Dark Tapestry yet... it's mostly mentioned in bits and pieces here and there.
Perhaps someday we'll do a bigger book about it.
I really hope you don't do a book on it. I don't think it needs any more info. We as DM have just enough info to make any nasty thing we want come from "The Dark Tapestry". Not that we cant do that if we have more information but I like to be able to DM between the lines and not every thing need to be laid out for us.
I'm not going to lie, the Dark Tapestry mystery for Oracles in UM confused the hell out of me. I had no clue where it came from until I searched around the forums.