In what way that question is nonsensical?
The Universe contains all things. Any thing that it would be expanding into would also be some thing that is a part of the Universe, and thus subject to that expansion. There cannot be anything outside of the Universe for which it to expand into.
Your answer seems to be a non-sequitor but the question is a most natural reaction to an unsupported and impossible claim made.
Well hang on, I'm not saying that the model is most definately the truth. I'm just explaining to you what the model says. Even if the model is wrong, we can still talk about what the model does and does not say.
To expand is a verb that has a certain meaning. A balloon can expand exactly because it can have a possible room it can be expanding into. The Universe is not a balloon nor is it a raisin muffin. It's lacking an oven needed to make a claim it does indeed perform the action alleged a plausible one.
That's fine, but all that really means is that the word "expand" is not totally sufficient for describing the behavior of the Universe in this model. All points in the Universe have the distance between them getting larger. That doesn't require something outside of the universe to exist.