The argument from contingency summarized by St. Thomas illustrates that in order for existence to be a reality there must be an "Uncaused Cause" that possesses existence in and of Himself. Everything else that exist does so by way of created contingency.
According to Mormon theology, God the Father is a physical being of "flesh and bones." Mormons identify him as the Biblical god Elohim. Latter-day Saint leaders have also taught that God the Father was once a mortal man who has completed the process of becoming an exalted being. According to Joseph Smith, God "once was a man like one of us and…once dwelled on an earth the same as Jesus Christ himself did in the flesh and like us."
According to Mormon scripture, "the elements are eternal". This means, according to Smith, that the elements are co-existent with God, and "they may be organized and reorganized, but not destroyed. They had not beginning, and can have no end." This was elaborated by Brigham Young, who said, "God never made something out of nothing; it is not in the economy or law of which the worlds were, are, or will exist."
Thus, Mormons deny ex nihilo creation and instead believe that God created or "organized" the universe out of pre-existing elements.
See MORMON COSMOLOGY
How does LDS cosmology address or reason with St. Thomas' contingency proof? In other words, according to the LDS, who or what causes existence to exist?