Consider the surface of a sphere. The sphere has a center, but it's not on the surface, so in a sense the surface has no center. If you find that unsatisfying,

Consider the surface of a sphere. The sphere has a center, but it's not on the surface, so in a sense the surface has no center. If you find that unsatisfying, you could posit that the universe does have a center, just not as a location in space that anyone could visit even in theory.