The comfort zone for atheists would be increased if it is certain that there is nothing, no feature, outside the observable universe.
You don't speak for atheists and have no idea what their, each being unique individuals, comfort zone is.
But the "where" question is broader if we are to accept the theorists that work with the dark energy of the void, energy that becomes part of the universe as it expands, the theory that there are other universes, in other space than ours, or the hyperspace that we are expanding into , moving into the future too.
That doesn't even begin to approach the concept of making sense.
to quote: "define nothing"-- how are thinly veiled ad hominem remarks helpful, compared to refuting specific assertions?
Thinly veiled? Interesting concept. Irrelevant and untrue, but interesting.
You've mentioned nothing. Please define that and give an example of "nothing".