Most of the matter in our universe is invisible. We can measure the gravitational pull of this “dark matter” on the orbits of stars and galaxies. We can see the way it bends light around itself and can detect its effect on the light left over from the primordial plasma of the hot big bang. We have measured these signals with exquisite precision. We have every reason to believe dark matter is everywhere. Yet we still don’t know what it is.
We have been trying to detect dark matter in experiments for decades now, to no avail. Maybe our first detection is just around the corner. But the long wait has prompted some dark matter hunters to wonder whether we’re looking in the wrong place or in the wrong way. Many experimental efforts have focused on a relatively small number of possible identities for dark matter—those that seem likely to simultaneously solve other problems in physics. Still, there’s no guarantee that these other puzzles and the dark matter quandary are related. Increasingly, physicists acknowledge that we may have to search for a wider range of possible explanations. The scope of the problem is both intimidating and exhilarating.
At the same time, we are starting to grapple with the sobering idea that we may never nail down the nature of dark matter at all. In the early days of dark matter hunting, this notion seemed absurd. We had lots of good theories and plenty of experimental options for testing them. But the easy roads have mostly been traveled, and dark matter has proved more mysterious than we ever imagined. It’s entirely possible that dark matter behaves in a way that current experiments aren’t well-suited to detect—or even that it ignores regular matter completely. If it doesn’t interact with standard atoms through any mechanism besides gravity, it will be almost impossible to detect it in a laboratory. In that case, we can still hope to learn about dark matter by mapping its presence throughout the universe.
But there is a chance that dark matter will prove so elusive we may never understand its true nature.
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