The universe itself is not the limit; there exist other planes of existence, dimensions, and realms from and to which adventurers, researchers, and all other sorts may travel, though the means are difficult to come by. Collectively, all these dimensions are known as the “Dimensional Cosmos” or, sometimes, the “Omniverse”.
Traveling between dimensions is a difficult thing, and few possess the means. On Earth, such technology deftly remains the realm of weird, fringe scientists and practitioners of the arcane.
The dimension in which Earth (and the rest of the known universe) resides is known as the “Material” plane. It is a uniquely large and stable dimension which has allowed it to exist for a very long time and develop rich histories across its innumerable galaxies.