Before adding the indefinite article A, a university refers to a university in general, and after adding the definite article The, a university refers to a university in particular.

If there is a preposition, or a preposition with a fixed collocation, and because universities can also be used as attributes, it is a common preposition for general nouns. When the beginning of a word is pronounced as a vowel, if the definite article in front uses a consonANt, it will be a, while the pronunciation at the beginning of the university is ju, so the first sound is J, not a vowel, so it will be A without an.