"The Universe is commonly defined as the totality of existence" but that is more of a philosophical position. Empirically we know (or believe) that it is of finite size some believe it is expanding others that is (or will be) contracting.
As we gain knowledge about distant places, our scope of area has expanded from Planet, to Solar system, to Galaxy, to Universe. In the same way we continue to find smaller and smaller building blocks, from Elements, to Atoms, to Quarks.
It is reasonable to assume, that the same gains in knowledge the removed the burden from Atlas, will find the edge of the Universe, and something that is beyond that edge.
What is the name of that which exists beyond the Universe?
Naysayers, who may claim that there is "nothing" beyond universe, are reminded that their beliefs are no less strong than, those who claimed that "nothing" was supporting the Turtle or the Elephant who like Atlas also supported the world. Lack of knowledge does not mean lack of existence.