The important thing in the balloon analogy isn’t what the balloon is expanding into, it’s just that every point on the balloon is drifting away from every other point.
One thing to consider, though, is that space may not even be a real physical thing. Maybe location is just a property of things, like mass or electrical charge. It could just be an inherent value that adjusts and influences other things according to the laws of physics. Maybe it’s less that “space is expanding” and just that “the location property of everything is constantly diverging.” There’s no need to worry about what anything is expanding into because our conception of space may just be a mental construct.
The important thing in the balloon analogy isn’t what the balloon is expanding into, it’s just that every point on the balloon is drifting away from every other point.
One thing to consider, though, is that space may not even be a real physical thing. Maybe location is just a property of things, like mass or electrical charge. It could just be an inherent value that adjusts and influences other things according to the laws of physics. Maybe it’s less that “space is expanding” and just that “the location property of everything is constantly diverging.” There’s no need to worry about what anything is expanding into because our conception of space may just be a mental construct.