It's Complicated
Ideas in science are often pretty simple. The details of the facts supporting these ideas rarely are. Like ancient civilizations, religions, and primitive cultures, modern science has constructed an elaborate cosmology. Modern cosmology, for example, has constructed a detailed picture of what was happening in the universe 13 billion plus years ago, when it was less than a millisecond old. Why should we take this cosmology, or any science, more seriously than the various myths aforementioned? The answer of science is that the scientific cosmology is supported by an intricate web of measurements, predictions, retrodictions, and interrelationships which are remarkably consistent, not only with each other but with the whole structure of modern physics, from thermodynamics to quantum field theory. That web is complicated, and nobody can comprehend it without extensive and detailed study. It's probably worth noting that perfect consistency is an ideal that is never perfectly achieve...