Axions, dark matter, and neutron stars: How to find the Universe's most mysterious matter

One of the most outstanding puzzles in all of astrophysics right now is simply stated: What the frak is dark matter*?

We know it exists and we know it's pretty much everywhere in the Universe, we know the kinds of things it does and the kinds of things it doesn't do… but, embarrassingly, we don't know what it actually is.

We have some good ideas, so scientists have been looking to see if any of these ideas pan out. In a recent paper, astronomers report looking for one particular flavor of dark...