Well, about the only answer I can come up with is that the universe is expanding outward as a result of the "bang" and the space between the point where the bang occurred and where the matter now sits, is just empty space like the inside of the balloon.
As far as the expanding balloon we are on, there is nothing beyond it but dark, empty space.........
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.