James' World 2

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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.

On a warm summer evening in August 2022, we huddled with a few other physicists around a table at the University of Washington. We were there to discuss the culmination of the “Snowmass Process,” a year-­long study that the U.S. particle physics community undertakes every decade or so to agree on priorities for future research. We were tasked with summing up the progress and potential of dark ­matter searches. The job of communicating just how many possibilities there are for explaining dark matter, and the many ideas that exist to explore them, felt daunting.

We are at a special moment in the quest for dark matter. Since the 1990s thousands of investigators have searched exhaustively for particles that might constitute dark matter. By now they’ve eliminated many of the simplest, easiest possibilities. Nevertheless, most physicists are convinced dark matter is out there and represents some distinct form of matter.

A universe without dark matter would require striking modifications to the laws of gravity as we ­currently understand them, which are based on Einstein’s general theory of relativity. Updating the ­theory in a way that avoids the need for dark matter—either by adjusting the equations of general relativity while keeping the same underlying framework or by introducing some new paradigm that replaces general relativity altogether—seems exceptionally difficult.

The changes would have to mimic the effects of dark matter in astrophysical systems ranging from giant clusters of galaxies to the Milky Way’s smallest satellite galaxies. In other words, they would need to apply across an enormous range of scales in distance and time, without contradicting the host of other precise measurements we’ve gathered about how gravity works. The modifications would also need to explain why, if dark matter is just a modification to gravity—which is universally associated with all matter—not all galaxies and clusters appear to contain dark matter. Moreover, the most sophisticated attempts to formulate self-consistent theories of modified gravity to explain away dark matter end up invoking a type of dark matter anyway, to match the ripples we observe in the cosmic microwave background, leftover light from the big bang.

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Olena Shmahalo

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