The only evidence that they had ever existed was an example of their food, saved by Annorax aboard his ship. This dish was served at a dinner prepared for Annorax, Chakotay, and Tom Paris in 2374. The species that made up this empire was restored when the weapon was erased from history. (VOY: "Year of Hell, Part II")
Since so little was said of this empire, it is unclear if the Alsuran Empire was an interstellar power or it was simply a group of nations on one planet. It is also unclear if the Alsuran Empire exists in the "restored" timeline of the 24th century.