The Nature of the Universe

There was some deep thinking going on at my pad yesterday.

"Mama," my nine-year-old son said.  "If the universe is flat, what would happen if you got to the end?"

"Well," I answered.  "If the universe is flat, then you'd fall off into a giant void."

"That doesn't make sense.  What if you lived on a planet that was right on the edge of the universe?  You'd be worried about falling into the giant void all the time!"  I thought that was pretty good reasoning coming from a kid his age.

"You're absolutely right, Bubba.  It's like in the days of Christopher Columbus when everyone thought the world was flat.  But it's not flat, is it?"

He shook his head.  "Then how does the universe look?"

"I think it's a big bubble," I said.  "You'll never get to the end.  If you could, you'd probably just go around and around and around."

He wrinkled up his eyebrows.  "But then what's outside the bubble?"

"Dark matter."

"Huh?"

"Yeah.  I don't get it either.  But if you study physics, you might figure it out.  And if you do, enlighten me!"

So what do you think?  Is the universe flat, or is it round?  And if it's flat, what happens if you get to the end?  And if it's round, what's outside of it?