The cosmological argument basically says something like: I have parents, and my parents had parents, and so on in time back to the beginning of the universe, asking how the materials that made up The Big Bang got there in the first place? The theist says we must posit God as creator to start the chain of causes. In fact, this theistic answer is a God of the Gaps fallacy, like the ancient Greeks not knowing why the sun went across the sky so they imagined the God Helios driving the sun across the sky. There is a gap in scientific knowledge regarding a precise scientific consensus about the very beginning of our universe, but as scientific and mathematical knowledge grows we can see we are certainly not at the point where a reasonable answer is that fairies created the universe. Here is an important video explaining why: