Dark matter is everywhere.  There is way more of it in the universe than the matter we are made of and interact with.  Yet for the sheer amount of it, we have no way of determining what exactly it is.  It’s as if we didn’t know what air was, and even though we could see it and breathe it, we couldn’t measure it.  The most tantalizing part about dark matter is that we can see the gravitational effect it has, and so we can determine how much of it there has to be.  Some places in the universe have more dark matter than...