If we agree on that, then the universe requires a cause, per premise 1 of the Cosmological Argument. This makes our dichotomy "Either the universe is created by itself, or the universe is created by something other than itself." Now, let's briefly grant it as a possibility that the universe is self-creating, rather than being created by an external cause (remember that this external cause must be without time or matter, since those both began at the big bang). When we test this theory against evidence, it doesn't hold water, because nothing else is self-created, so it's incredibly unlikely (in the absence of some other overwhelming evidence) that the universe is self-created. God, on the other hand, is self-existent, but not self-created, because He was never created, because he (again, by definition) never began to exist. That is the meaning of "eternal" or "atemporal".
To state it another way, something has to be eternal, or you have infinite regress (y was caused by x, which was caused by w, which was caused by v, which was caused by u, literally ad infinitum, because each of them, not being eternal, began to exist). Now, by Ockham's Razor, it is simpler for there to be one eternal thing and one non-eternal thing than for there to be a chain of non-eternal things that caused each other, and then one eternal thing. Consequently, the simplest possible arrangement is for there to be one eternal, atemporal Cause, which then caused the universe.
Now, just hypothetically, the one "eternal thing" could be a "ray" rather than a "line"; that is, it could extend back infinitely far, but have stopped at a given point, which would give you a deistic universe, in which a Cause (I won't call it God) created the universe and then ceased to exist. But that's completely irrelevant to our debate.