One presentation of this argument is put forward by Carl Sagan:
"If the general picture, however, of a Big Bang followed by an expanding universe is correct - what happened before that? Was the universe devoid of all matter and then the matter suddenly somehow created? How did that happen? In many cultures, a customary answer is that a "God" or "Gods" created the universe out of nothing, but if we wish to pursue this question courageously we must, of course, ask the next question - where did God come from? If we decide that this is an unanswerable question, why not save a step and conclude that the origin of the universe is an unanswerable question? Or if we say that God always existed, why not save a step and conclude that the universe always existed? There's no need for a creation, it was always here. These are not easy questions. Cosmology brings us face to face with the deepest mysteries, with questions that were once treated only in religion and myth."
Source: https://genius.com/Carl-sagan-on-god-and-gods-annotated
Or watch: The uncertainty of God (Carl Sagan in cosmos series) - YouTube
Richard Dawkins makes similar arguments:
"If we want to postulate a deity capable of engineering all the organized complexity in the world, either instantaneously or by guiding evolution, that deity must have been vastly complex in the first place. The creationist, whether a naive Bible-thumper or an educated bishop, simply postulates an already existing being of prodigious intelligence and complexity. If we are going to allow ourselves the luxury of postulating organized complexity without offering an explanation, we might as well make a job of it and simply postulate the existence of life as we know it! The Blind Watchmaker, Chapter 11 “Doomed Rivals”" (p. 316)
"A God capable of continuously monitoring and controlling the individual status of every particle in the universe cannot be simple. His existence is going to need a mammoth explanation in its own right." The God Delusion (p. 178)
"God, if he exists, would have to be a very, very, very complicated thing indeed. So to postulate a God as the beginning of the universe, as the answer to the riddle of the first cause, is to shoot yourself in the conceptual foot because you are immediately postulating something far far more complicated than that which you are trying to explain. [...] If you have problems seeing how matter could just come into existence - try thinking about how complex intelligent matter, or complex intelligent entities of any kind, could suddenly spring into existence, it's many many orders of magnitude harder to understand." Lynchburg, Virginia, 23/10/2006
"In the case of the cosmos, [...] even if we don't understand how it came about, it's not helpful to postulate a creator, because the creator is the very kind of thing that needs an explanation - and although it's difficult enough to explain how a very simple origin of the universe came into being - how matter and energy, how one or two physical constants came into existence - although it's difficult enough to think how simplicity came into existence, it's a hell of a lot harder to think how something as complicated as a God comes into existence" "Has Science Buried God?" Debate, Richard Dawkins vs. John Lennox, 21/10/2008
How do Christians respond to the atheistic argument that postulating a God introduces an unnecessary and overly complicated extra step?
Note: there is an ongoing related discussion taking place on Philosophy Stack Exchange right now, Is God’s very existence the ultimate miracle?