Its possible the universe is just one giant black hole and theres other universes that are other giant black holes, each with its own unique laws of physics, gravity etc. And they are all just infinitely expanding. There's so many theories out there its kind of pointless to sit and wonder for anything other than entertainment purposes. Unless you're a scientist who gets paid to do so, then this is totally your bag. Personally I think we're nothing more than the arteries and organs and molecules and cells of some giant interstellar being, and other "universes" are just other beings. I mean, if you think about it, if you asked a cell in your body how big the universe was, it would probably measure the universe as being your body and wouldn't know much about anything beyond that. So who's to say the same isn't true on a larger scale. Again, stuff like this is fun to think about, half the crazy science fiction stories people have come up with over the years have been people thinking about crazy stuff like this.
Also, i would never call the universe dark. Dark is a concept humanity came up with to give some understanding to the concept of empty space, at least...empty as we know it, its very possible that just because we think a particular part of space is empty doesn't mean it actually is, its just empty to us. Plus, if you look at the universe with anything other than visible light, it suddenly becomes a lot less dark.