Overview: Quantum mechanics is our best theory of the fundamental nature of reality, but it's usually only distinguishable from familiar classical mechanics on the smallest scales. However, there are some fringe cases where its distinct features manifest on scales we can observe—in things like superfluids, or the interiors of collapsed stars. But it’s also possible that our entire galaxy is filled with a reverberating quantum mechanical wave that literally holds the galaxy together—and in fact explains all the dark matter that we see across the universe. And this isn’t even a fringe theory. It’s axionic dark matter.