Remember, the balloon is a 3D visualization of a 4D world [ours.] So we must remove one dimension -- and keep in mind that we removed one dimension -- in order to produce the visualization.
Our world: 3-space, 1-time dimension. Balloon world: 2-space, 1-time dimension. The time dimension in the balloon world is represented by the radius of the balloon. As the balloon expands, the 2 spatial dimensions on the skin of the balloon are evolving in time [moving farther apart.]
That is the answer to your question.
Inside the balloon is the past. "Inside" the 4 dimensional hypersurface of our universe are all of our yesterdays. Outside of the balloon is what it is evolving into: the future. And that is what is "outside" of the four dimensional hypersphere of our universe is our future.
You are correct [as far as we know] that there is a barrier that keeps us from going "outside." And there is also a barrier that keeps us from going "inside" as well. We cannot visualize it, but we know that we cannot travel through time.
That barrier has a name: Entropy.
I appreciate the answer but I'm still unclear about the "barrier".
Are you saying that the "barrier" that would prevent us from viewing or knowing what lies beyond the edge of the universe is "time" in the sense that we would never have enough time in which to actually travel to that edge?
Or is there another explanation that I can't seem to grasp?