Imagine all the whole numbers from 0 to 10. Now imagine imagine every half number in that span. Keep adding the set of numbers between the numbers you already have. The size of your set is expanding, but the bounds of the set itself need not expand. You can imagine a similar concept to an infinite set of numbers, and thus you can imagine the universe expands in a similar fashion.
JustKiddin9 t1_jedar7x wrote
Reply to ELi5 If the universe is expanding, what is it expanding into? by cashmoneyhash
Imagine all the whole numbers from 0 to 10. Now imagine imagine every half number in that span. Keep adding the set of numbers between the numbers you already have. The size of your set is expanding, but the bounds of the set itself need not expand. You can imagine a similar concept to an infinite set of numbers, and thus you can imagine the universe expands in a similar fashion.