It was a major fail. There I was, a naive undergraduate, waiting
in my professor’s office as he spoke with an older student about
some theoretical problem that wasn’t working out. After the
student left I boldly asked him: “Couldn’t you just redefine
everything to make it come out OK?” The withering look that
followed told me all I needed to know about how really stupid my
suggestion had been. Rewriting X as Y in all the equations
wasn’t going to help anything. But what are the options when a
scientist faces a problem with no obvious solution? That is the
dilemma astronomers have faced over the last few decades as they
took a census of cosmic matter and motion. Mapping the beautiful
pinwheel arcs of spiral galaxies, they found the constituent
stars moving far too fast to be explained by the galaxies’ known
mass. All matter produces a gravitational force that tugs
surrounding material into motion. But summing up all the matter
they could see in the pinwheels left astronomers with far too
small a reserve to exp