Eli5 if the universe is everything but it is constantly expanding, what is it expanding into?

Question

Eli5 if the universe is everything but it is constantly expanding, what is it expanding into?

In: 22

Answer 1

I read somewhere that the space between planets, stars etc is stretching further apart. Like raisins in dough, when you put it in the oven the dough will rise into cake but the raisins are farther apart now.