If the universe had existed always, then it would be in a state of heat death by now. As we can see, it is not. Therefore, the universe has not existed always.
I did not know this, but I'll take your word for it.
I understand what you're saying.
The choices are:
1. the universe came into being "on its own" somehow.
2. a "god" (eternal something or other--"eternal" meaning outside of space-time) caused the universe to come into existence.
Both of these possibilities strike me as equally strange and magical.
Therefore we can say that it is just as rational to believe in either option. Both are on the same level of strangeness.