The answer is similar to the one I gave for your other question: https://news.ycombinator.com/item?id=24424912 https://news.ycombinator.com/item?id=24424912 B

The answer is similar to the one I gave for your other question: https://news.ycombinator.com/item?id=24424912 https://news.ycombinator.com/item?id=24424912 But I'll give you another analogy anyway. Say you put two rocks into a mass of water in an environment without gravity (e.g. the ISS), then freeze the water, causing it to expand. The expanding ice will cause the rocks to become more distant from each other, but they're not "moving" in local terms - the manifold in which they exist is itself expanding. That said, the frozen water and the rocks both exist in space, and the expanding ice moved the rocks through space. But, in this metaphor, there is no second, external "space" - the water itself represents space. And so to for our universe. Space itself isn't a part some other, second "space" that it's expanding into (at least not so far as we're aware). The existence of somewhere for space to expand "into" is not necessary for the expansion to take place. Remember, it's the scale factor which is changing, not a discrete object (i.e. the universe) physically expanding.