If the universe is/was infinite, it would have no end nor beginning; it would have just 'always been there'... But infinity is not possible, nor real, it is an irrational number usually used for/as an expression or exaggeration of something. So the notion of an infinite universe can be ruled out.
On the other hand, if the universe is not infinite, what lies outside of it? Because, just like the 'number' infinity, the physicality of 'nothing' is not real and can not exist either, but something has to lie outside of it, because as I just stated, 'nothing' can't be outside of it, 'something' has to be.