According to the Mormon doctrines, who created the universe? This may be related to a previous question about the first god who began eternal progression.  This is something I've really been wondering for a while.

If the Almighty God began life as a man, he must have been part of this universe.  As part of this universe, it would've been impossible to create the universe--since that requires a deity that it outside the universe.

Do Mormons believe that the universe existed before God?  Or that God existed as God before the universe began (and therefore is one who didn't ascend to godhead, but was always a deity)?

In essence, either the universe always was and there was a first god who created everything (related to this question) or there is a god who existed, as god, before the universe.  Yet the Mormons teach that God was once a man.  So, who created the universe?
Mormons do not believe in creation ex nihilo, as traditional Christian doctrine does.  So to ask who created the universe in Mormonism is asking the wrong question.
Rather, Mormons believe that god arranged pre-existing matter in creation. The matter always existed; creation was an act of organizing the matter.
From Ancient Views of Creation and Doctrine of Creation Ex Nihilo by Stephen D Ricks:

To these should be added his[Josephs Smith's] powerful rejection of creation ex nihilo (&quot;from nothing&quot;)—according to which doctrine God alone is eternal and uncreated while matter is not—and his affirmation of creation from preexisting matter.
Abraham 3:24 in the Pearl of Great Price confirms the Latter-day Saint belief in creation from preexisting matter: &quot;And there stood one among them that was like unto God, and he said unto those who were with him: We will go down, for there is space there, and we will take of these materials, and we will make an earth whereon these may dwell.&quot;

This is a clear distinction between the beliefs of Joseph Smith and conventional Christianity. Mormonism affirms that physical material is eternal and organized by God; conventional Christianity affirms that God is eternal and that He created the physical world out of nothing.