Dark matter is one of the oldest open problems in physics. It makes up most of the matter in the universe and shapes how galaxies form and move, yet no one has ever directly detected it. Physicists have proposed dozens of candidates to explain it: exotic new particles, black holes formed moments after the Big Bang and more.

With no direct detection to settle the question, researchers often fall back on a different test: not “is this candidate detected,” but “is this candidate natural?”

Naturalness is physics’ version of Occam’s razor. A theory is called natural if it explains the universe we see without requiring its underlying numbers to be delicately, almost implausibly, fine-tuned. A theory that only works if several unrelated quantities happen to cancel out to many decimal places is treated with suspicion, even if it isn’t strictly ruled out.