I can’t fully grasp the expansion of the universe. It seems intuitive to me that if something expands it must be expanding in something larger. Back when I was going to major in physics one professor told me to imagine a balloon expanding as a way of thinking about the expansion of the universe. It didn’t help me. A balloon can only expand since it is in a larger structure with room to expand.<p>So what is the universe expanding in? If this isn’t a valid question then how should I try to understand the universe expanding?