Aristotle's unmoved mover was created primarily as a way to explain planetary motion, not existance. The unmoved mover didn't create anything, according to Aristotle, as accoring to him everything that exists has always existed. Of course, this is false, as we know about evolution and have seen in human history many species go extinct.
More than that, he even thought that there may be more than one unmoved mover for each planetary sphere. The greek astronomers of his time believed that each planet moved on its own sphere, combining to make a mechanical contraption that moved the heavens. Aristotle believed this contraption is set in motion by the planets' love for the unmoved mover, or at least the UM for their sphere. Yes, that's right, Arisotle's answer to what moves the planets? Love. Aristotle was brilliant, but even geniuses can believe things that are utterly stupid.