This image of the galaxy cluster Abell 520 — 2.4 billion light-years away — may challenge some of our basic theories about Dark Matter. It is overlaid with false-color maps showing concentrations of starlight (orange), hot gas (green) and mass (blue). The mass is mostly Dark Matter.
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 explain how fast the galaxies were spinning.
Soon, the same problem appeared in other cosmic quarters. Giant clusters of galaxies showed the same kind of mass deficit. Even data from cosmology — the study of the universe as a whole — pointed to some mismatch between what we can see and what's out there.
Something else, it appeared, was going on and astronomers were on the hook to come up with an explanation.
At that point they had two obvious choices. The first was changing the laws linking gravity and matter. The second was to invent a new form of mass that was entirely invisible but still gravitationally "active."
The second choice — the "discovery" of Dark Matter — became the consensus path of modern astronomy. To many folks, Dark Matter might seem like a desperate fiction or dodge. The truth is, Dark Matter was the easy choice.
It might seem simple to just change the known laws of nature to account for some new puzzling phenomena. The reality is that those laws have been tested a thousand times in a thousand interlocked ways. Make a little shift to account for galaxy spins and a hundred other consequences pop up. If they are not wacked down, the whole tapestry of physics unwinds.
Some scientists, like the brave Mordehai Milgrom, did try altering Newtonian Physics to account for galaxy spins without adding Dark Matter. But the complications needed to account for this change have kept most physicists from jumping on Milgrom's Modified Newtonian Dynamics (MOND) bandwagon.
Was that path a mistake? It's not impossible that Dark Matter was the wrong choice. Perhaps, waiting down the road, there is a new physics that will make Dark Matter vanish like the scary shadows in a child's darkened room disappear when the light switches on. That might still happen. But, in truth, we have gotten pretty far down that second path now and it has widened into a well-paved road with every new discovery that Dark Matter neatly explains.
At some point a choice always has to be made. Science will follow that path until it dead ends or opens up new avenues of explanation. That's what discovery is all about.