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Five-sixths of the matter in the universe is a mystery to us. Now, we might have taken a step closer to figuring out what it is. Maybe.

On September 1, the team running the LUX-ZEPLIN (LZ) experiment searching for dark matter reported a development at a conference in Tendo, Japan. Nearly a mile underground in an old South Dakota gold mine, the team watches a tank containing ten tons of ultra-pure liquid xenon. Going through 220 days of data collected in 2023 and 2024, they found a flash of light—the signature of a xenon nucleus recoiling with 248 kiloelectronvolts of energy it picked up from a source no known background phenomenon explains. They calculated the significance of the event at 2.6 sigma—meaning the background would throw off an event like this roughly once in two hundred tries. That isn't enough to call the detection a "discovery," which typically requires five sigma or higher. They didn’t claim to have seen direct evidence of dark matter. But they saw something interesting.1If LZ, or another experiment, repeats this finding, it could have major ramifications for astrophysics and cosmology. Dark matter out-masses all the rest of the matter in our universe at a ratio of about five to one,2 and yet we don’t know what it is.

In 1933, Swiss astronomer Fritz Zwicky determined the galaxies in the Coma cluster were moving too fast for the cluster’s visible mass to hold them together. Something else—he called it dunkle Materie3—had to be supplying the necessary gravity. The astronomy community basically ignored his finding for 40 years.4 Then in 1970, Vera Rubin and Kent Ford measured how fast the outer regions of the Andromeda galaxy rotate and found the same problem—the edges were turning nearly as fast as the middle, which Newtonian gravity flatly forbids unless the galaxy’s mass is distributed in ways that don’t match what our instruments detect.5

Fast-forward to 2006, when a team led by Douglas Clowe mapped a pair of galaxy clusters caught mid-collision. The gas in the clusters—which is where nearly all the ordinary matter lives—had piled up in the middle, slowed by the impact. But gravitational lensing showed the center-of-mass to be separated from the center of the gas by a significance of eight sigma. They called their paper: “A Direct Empirical Proof of the Existence of Dark Matter.”6

The cosmic microwave background provides additional confirmation. The Planck satellite’s measurements fix cold dark matter at roughly five times the density of ordinary matter. Everything we can identify—gas, planets, nebulae, stars, galaxies, quasars, etc.—amounts to about a sixth of the material universe.7 Everything else is invisible and intangible, detectable to us only by its gravity.

So, what is it?

Maybe it’s not invisible, just really hard to see…

Before we dive into speculative explanations, let’s address the simplest possibility. Perhaps dark matter isn’t really exotic; it’s just faint—burnt-out stars, drifting planets, stray black holes. These are often called massive compact halo objects (MACHOs). Anything dense enough passing in front of a background star will briefly brighten it by gravitational lensing, so the idea is directly testable.8

The EROS-2 survey watched millions of stars in the Magellanic Clouds for 6.7 years. If the Milky Way’s halo were made of objects around four-tenths of a solar mass, roughly 39 lensing events should have shown up in the bright-star sample—only one did.9 The cosmic microwave background and Big Bang nucleosynthesis independently cap the total amount of ordinary matter far below what the galaxies' gravity demands—no matter how faint that matter might be.10Whatever holds the galaxies together, it isn’t made of the same stuff we are.

Science fiction is on the case. Three novels in particular—Mike Brotherton’s Spider Star, John Gribbin’s The Alice Encounter, and Stephen Baxter’s Ring—each take a different approach to dramatizing the problem.

WIMPs to the rescue?

If dark matter isn’t just ordinary matter that’s hard to see, maybe it’s a particle no one has found yet.

The favorite candidates for decades have been “weakly interacting massive particles” (WIMPs)—hypothesized to be heavy, stable particles produced in abundance in the early universe and left over when the cosmos thinned out.11 But we’ve hunted for a long time and nothing suitable has turned up.

LZ published its low-mass results last December, based on 417 live days—the largest dataset any dark matter detector has ever assembled—and found nothing between three and nine proton masses.12 Meanwhile ADMX is searching for another, much lighter, proposed particle called the “axion”, with no success to-date.13

Mike Brotherton is a working astronomer, as well as a science fiction writer, and knows his physics. His novel Spider Star, a WIMP story, includes a planet made of weakly interacting matter, and a starship drive that scoops and pushes WIMPs as fuel14—a fascinating premise, and possibly even realistic, but as of now, still hypothetical.

Maybe it’s a whole second world

We have no good reason to conclude only one type of particle comprises dark matter. Ordinary matter has a dozen fundamental particles, multiple forces, and a periodic table. Dark matter could have the same—dark electrons, dark electromagnetism, dark chemistry—coupled to us by gravity and almost nothing else.

The most elegant version is mirror matter, in which dark matter adheres to an exact copy of the Standard Model, with every particle getting a mirror twin.15 Earlier this year, theorists published predictions for electromagnetic signatures of mirror stars which have captured ordinary atoms.16Mirror matter is the driving conceit behind John Gribbin’s The Alice Encounter. Gribbin, an astrophysicist by training and a popularizer by trade, builds his novel on “Alice matter.” This is "looking glass" stuff, much more plentiful than normal matter, arranged in a roughly spherical distribution around the flat disk of our galaxy. Beings composed of Alice matter, and more advanced than we are, have noticed the “contamination” of normal matter in the universe and have decided to investigate.17

Inside the Sun?

Whatever dark matter turns out to be, if it interacts with ordinary matter at all, some of it should end up in the cores of stars. Stars like our sun sweep up dark matter as they move through space, and a particle that scatters off a nucleus of normal matter along the way can lose enough speed to be stranded. Gravity does the rest, settling the captured particles into the core.18

In Stephen Baxter’s Ring, the fourth novel in his Xeelee series, the downloaded mind of a woman named Lieserl is lowered into the Sun and abandoned there for five million years. She finds the star inhabited by “photino birds.” These are dark matter creatures that colonize stellar cores, drain the energy that drives fusion, and age the stars prematurely into white dwarfs incapable of killing the photino birds in supernovae.19

The “photino” is the proposed supersymmetric partner of the photon, and in the 1980s it was among the leading candidates for cold dark matter.20

Maybe there is nothing there at all

The unfashionable possibility is that the discrepancy is real and the interpretation is wrong. Perhaps gravity behaves differently at very low accelerations, and no missing substance is required. Mordehai Milgrom’s Modified Newtonian Dynamics (MOND), proposed in 1983, fits galactic rotation curves with a single new constant.21 MOND detractors point to the work on colliding galaxy clusters by Douglas Clowe and his colleagues mentioned above, but MOND’s advocates were still publishing rebuttals earlier this year.22

Stories waiting to be written…

Cold Company. If dark matter can lose energy, some of it may have settled into a thin disk lying in the plane of the galaxy, which the Sun crosses roughly every thirty million years.23 A geologist and a physicist argue for decades about whether the impact record is a coincidence or a timetable—and about which of them will have to be wrong in public.

The Long Fog. Due to something called the “neutrino fog,”24 dark matter detectors hit a wall no practical modifications can bypass. The answer lies permanently below the noise. A story about the last generation of experimentalists, what a scientific culture does when it proves a question unanswerable, and those who refuse to accept this limitation.

Alice’s Argument. An homage to Isaac Asimov’s The Gods Themselves,25 further exploring one of that book’s most provocative premises. A laboratory converts a gram of ordinary matter into mirror matter, then converts it back. But it returns changed—edited. Someone made of mirror matter is sending us a message. First contact must be conducted a gram at a time, in a medium where no one can verify anything, and the negotiators on our side cannot agree on whether the other party is a government, a machine, or a child.

What comes next

The Vera C. Rubin Observatory began its ten-year survey of the southern sky this summer, and will map dark matter, in part, by the way it bends light across the visible universe.26 The Nancy Grace Roman Space Telescope launched on August 30 and is less than three months from its post at L2, where it will pursue a similar objective.s Underground, the LZ team keeps working, waiting to learn whether their recent detection is repeatable.

Maybe one of the greatest unsolved mysteries of science won’t remain unsolved for long.

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