Whatever the topic happens to be, if the answer to the question is "We do not know", that is not a valid reason to shoehorn in the unsupported answer of your choice. This is presuming that a god is the only answer to the question.
The burden of proof is always on the person making a claim - no matter what the claim is.
"God did it." is not the default answer.
On a subject where the answer is not known, the procedure is to investigate, and assemble an evidence-based answer - not conjure an answer out of thin air.
This question arises as a common format, with whatever topic inserted into the blank.
Whatever the topic happens to be, if the answer to the question is "We don't know", that is not a valid reason to shoehorn in the unsupported answer of your choice.
For thousands of years, humans have been positing supernatural explanations for phenomenon, ranging from why the sun goes across the sky, to what causes earthquakes, to why lightning strikes, to what's making a mentally diseased person behave oddly.
At some point, someone asked, "If there's no god, what causes the Arora Borialis?", "If there's no god, what causes the sun to rise each day?", "If there's no god, what causes lightning?"
For thousands of years, humans have been pragmatically and analytically investigating these phenomenon, throughly and methodically.
One after another, each of these supernatural explanations have given way to objective reality, as we discover that each is actually just another naturally occurring event.
We have never confirmed any supernatural claim.
Given that history, one should perceive a pattern. If one is interested in accurately gathering knowledge, the best approach is to not accept claims as true until they've been sufficiently objectively verified with good solid evidence.
The burden of proof is always on the person who is making a claim. When a person posits a supernatural being as the explanation to something that we otherwise do not understand, yet, he or she thus has the burden to demonstrate that claim as true.
Many theists assume that the default answer is a god. History has shown that to be an inaccurate assumption, over and over and over again.
When atheists say, "We do not know", the answer is, "We do not know", not "God did it." or "Morginsplat did it." or "A giant Siamese purple and green polka-dotted unicorn did it."