The correct form is "the universe." In English grammar, the definite article "the" is used before the word "universe" because it refers to a specific, singular entity—the entirety of space, time, matter, and energy that exists. "The universe" is a fixed concept, and it is considered a proper noun.
On the other hand, "a" is an indefinite article used before singular nouns to indicate any one of a group, and "an" is used before singular nouns beginning with a vowel sound. However, since "universe" does not begin with a vowel sound, "an universe" is not correct.
So, when referring to the entire cosmic entity encompassing everything, we use "the universe."