Fritz Zwicky knew that something fishy was going on.

It was the early 1930s, and in the years following Hubble's spectacular and surprising result—that galaxies are things and that they are, on average, redshifting away from us, implying that we live in an dynamic, expanding universe—astronomers had undergone a quick change in heart. With gigantic new telescopes like the hundred-incher on Mount Wilson, which Hubble used as a massive eye to peer beyond the limits of the Milky Way, astronomers went from debating the very existence of extragalactic objects to racing to catalog as many of them as possible.

If you can't beat ’em, join ’em, I guess.

Now in survey mode, astronomers reclassified known spiral nebulae as spiral galaxies (although the term “nebula” would still persist for some time, thankfully we've been able to drop that anachronism by now; otherwise our discussion of cosmology would be even more saddled by the presence of yet another historical-jargony albatross) and started mapping the heavens for as many deepest-sky objects they could find.

Zwicky, the Swiss astronomer renowned for his acumen, creative (and sometimes crazy) thought processes, and prickliness, took a special interest in a group of galaxies in the direction of the constellation Coma Berenices.

Now, even the most ardent astronomy enthusiasts will admit that there's basically nothing interesting happening in the constellation. Heck, even its name, Berenice's Hair, doesn't exactly inspire the wonder and majesty we typically associate with the night sky. There are a few Messier objects, but otherwise it's an unremarkable patch of darkness.

Unless you look deeper. The universe in that particular piece of the sky is swarming with galaxies, hundreds of them, much more than other random directions in the sky. That itself is an intriguing fact to note, which astronomers in the first half of the twentieth century surely did: Galaxies outside the Milky Way are not scattered around randomly and uniformly. No, there are vast empty patches and what appear to be clusters of these galaxies. The astronomers at the time had no idea of what to make of this, but as we've already begun to see in the previous chapter, the solution will become apparent to them soon enough.

Zwicky diligently mapped, measured, and cataloged as many galaxies in one cluster in particular, the Coma Cluster, as he could; a veritable cosmic butterfly case of extragalactic creatures. All those galaxies had roughly the same distance from our home, which was the first clue that it wasn't an accident of optics that led to their clustering in the sky. No, these galaxies were associated with each other in deep space—they lived together.

Since the galaxies lived together, Zwicky guessed that this cluster must be stable. If you saw a random group of people at a random time of day at a random house, you might guess that those people aren't total strangers collected together by pure chance—they're probably a family. They've been together for a while. Sure, it might just be a guest-filled house party on a Thursday afternoon, but it's not likely. This argument is from statistics, and it's a pretty useful one in cosmology. Structures that don't last long (in cosmological terms) simply won't persist long enough for us to catch their light in the small window of time that we've been observing the deep heavens.

It's a safe assumption: if you see something, it's already been there a long time.

Zwicky cleverly used that assumption to start answering a very simple but deeply difficult question: how much does that cluster of galaxies weigh? How massive is it? It's one of the most straightforward questions we can ask of anything, even celestial objects. It's high on the standard list of questions for describing anything, really. You get a baby announcement, and you get a few key pieces of information: name, sex, length…and weight.

Astronomical objects aren't gendered, so as soon as we name something (like, say, the Coma Cluster), our next job to break out the rulers and scales. Which is hard, because things in space are typically gigantic and far away. But Zwicky used an old physics trick known as the virial theorem. Originally developed in the nineteenth century, it connects kinetic to potential energies within a system of particles bound together.

We'll jump right to Zwicky's application of the theorem to explore what all that jargon means. First off, the “particles” are going to be entire galaxies. Yes, galaxies are gigantic, but they're peanuts compared to the size of a cluster. The kinetic energy is related to the speed of every galaxy—the faster the galaxies are buzzing around, the greater the kinetic energy. The potential energy here is provided by gravity—the mutual interaction of all the galaxies provides the glue holding them together.

This is where stability matters. The galaxies are pulled together by gravity, by the invisible strings (or deformations in space-time, if you're feeling relativistic) that insist on bringing objects closer together. Resisting that is the intrinsic velocities of the galaxies themselves. You can imagine a cluster of galaxies like a tremendous swarm of bees. Gravity pulls the swarm into the space of a sphere and really, really wants to make that sphere smaller, but if the bees have enough energy, they'll just keep buzzing around inside that sphere.

If gravity overwhelmed the kinetic energy, the cluster would have collapsed a long time ago. If the galaxies were too energetic, it would have exploded. Since the time needed to collapse or explode is short (again, just as a reminder, that's “short” compared to things like the age of the universe), it's a good guess that these forces are in balance.

This allows a person like Zwicky to make a relatively easy measurement (the average speed of the galaxies is given by their redshift) and convert that into a relatively hard measurement (the mass of the cluster). So he did.1

He got a number that was a bit too large, honestly. He knew how much mass was in all the stars surrounding the solar system, and he knew how much light they produced. So that gives a pretty simple formula: if you see this much light, then you can make a good guess about how much mass is generating that light.

But that number, based on counting all the hot, glowy stuff on the Coma Cluster, didn't agree with the number produced by the virial theorem. The virial theorem was based on simple kinematics; it was just motion that was affected and balanced by gravity, which was on pretty solid footing. But that calculation, based on physics as good as anybody's, resulted in a cluster mass five hundred times larger than you would have guessed by adding up all the light sources.

Uh, what? What could explain this discrepancy? Maybe stars in the Coma Cluster are weirdos and don't behave like any other star anywhere else. That doesn't seem to fit. Maybe the laws of physics are different way out there than over here. Kepler surely wouldn't appreciate that, but he's dead and can't complain, so we'll keep that in our back pocket. Perhaps the Coma isn't in equilibrium after all—maybe we caught it in a really awkward time in its life and the ground rules of the virial theorem don't apply. Maybe, but while possible, that's unlikely.

Maybe it's something else. Maybe there's material in the Coma Cluster that isn't all hot and glowy. There could be macroscopic or microscopic stuff that we simply aren't seeing. In the words of Fritz Zwicky himself, maybe the cluster contains vast amounts of dunkle Materie.

It's German, and I'll translate for you: dark matter.

It was about time for a shake-up anyway. While astounding and capable of bending even the biggest of brains, the observational results of Hubble coupled with the theoretical insights of Einstein and company at least made sense when taken together: we live in an unfathomably large universe that's getting fatter every single day. We may not like that answer, but at least it's an answer.

But Zwicky's dark matter wasn't an answer—it was a question. Other clusters were tagged and bagged by later astronomers, and they each showed this curious disagreement between different measurements of the mass. But while it was noted, nobody made a serious move to try to explain it. I can't blame anyone: if you were an astronomer interested in cosmology, there was all sorts of excitement about the big bang, the cosmic microwave background, and the creation of elements to pass your time. No need to worry about tiny niggling issues like giant discrepancies in cluster mass estimates, right?

And so Zwicky, and everybody else, moved on from the mystery of dunkle Materie to other, more pressing and interesting problems. A generation went by without much further thought on the subject. Then in the 1970s another astronomer, Vera Rubin, saw the same problem crop up in another situation, and what was considered a forgotten yeah-we-should-get-around-to-that-someday problem rose from the dead, setting the stage for one of the great enduring mysteries of the modern cosmological age.

The brief window of explaining and understanding the universe in a consistent, coherent story was quickly closing. Oh well, it was nice while it lasted.

Rubin was studying the motions of stars in other galaxies, which by any measure (and especially mine, since I'm writing the book) is an amazing feat of human curiosity. Just a century ago, we were struggling to identify the distances and motions of the stars right in our own galactic neighborhood, and by the late 1960s, Rubin was investigating, with great success, detailed interior movements in structures millions of light-years away.2

It's a pretty straightforward measurement once you get the hang of it. Pick a galaxy. Zoom in on various parts of it. Look at the spectrum. Identify some known elements. Measure the shifting of the spectral lines associated with those elements relative to what you know on the Earth. The same song and dance from a century before repeated over again, just at unimaginably large distances.

In the case of the entire galaxy, you can apply this technique to find that the whole structure is moving, and usually you'll find that it's in a direction away from us. That's what Hubble found. But if you focus your scope on individual bits of the galaxy, you'll pick up some extra motions on top of the general movement. If it's rotating (and, uh, they do) then one side of the galaxy will be slightly blueshifted—spinning toward us—while the opposite side will be a little bit redshifted—spinning away from us. You can repeat this exercise at different spots in the galaxy and build up what's called a rotation curve: the orbital speed at various distances in a galaxy away from its galactic center.

I won't hold back the plot twist: Rubin didn't find the speeds she was expecting. I know, shocking.

Here's the deal. When you just look at a spiral galaxy, and I really mean look, you'll notice that there's a big bulge of material (stars, gas, etc.) in the center, with a relatively thin and empty disk surrounding it. And our dear old friend Kepler can tell you what the orbital speeds of stars out in the disk ought to be. OK, fine, it's really Newton's universal gravity, but it was Kepler who first spotted the harmonious motions in the planets, and universal gravity being universal and all, the same method applies on these gargantuan scales. The speed of an orbiting object, whether a planet in the solar system or a star in a distant galaxy, depends on its distance from the central gravitating body and the mass of that body. A bigger sun = faster planetary speeds. Closer orbits also = faster planetary speeds.

Great, lovely. If we apply this to a galaxy, where most of the material is obviously concentrated in the center, then we ought to find that stars nearest the center orbit the fastest, with a general laziness setting in as we move farther out in the galactic disk.

But alas, no. Rubin saw something completely different: totally flat rotation curves. Stars out in the boondocks, at the very edge of civilization, were orbiting just as fast as their more centrally located cousins. The stars in galaxy are simply moving too fast. There isn't enough gravity to contain motions of that speed—the grand, beautiful spirals that we know and love should have flung themselves to pieces long ago, not stayed glued together for eons.

And this wasn't just the case in one galaxy, but across dozens. So, unlike with Zwicky's cluster, we can't appeal to a chance fluke of observations. Even if one of these galaxies were simply acting strangely, most of them are in equilibrium. Apparently, moving too fast for the gravity of your own galaxy is just the natural state of things.

And in the decades since Zwicky's initial proposal, we had much more confidently pinned down the relationship between stellar output and mass. Even accounting for the mass of nebulae, whether they glowed brightly or not, there wasn't enough stuff inside a galaxy. Once again, one measurement (counting all the hot and glowy bits) was disagreeing with another measurement (one based on motions). Zwicky saw it in a single cluster, and the world ignored it. Rubin saw in galaxies everywhere she looked, and the world started paying attention.

We're at a crossroads. Nature is not playing fair. Different measurements of the same quantity—answering the simple question of the total mass of a galaxy or cluster—were revealing different answers. And not just by a little bit. At best, the amount of glowing matter in a galaxy or cluster, even accounting for all the wave bands from radio to gamma rays and all the possible sources from nebulae and stars to brown dwarfs and giant black holes, was about one-fifth the value required by other mass estimates.

Rubin's result was as simple and annoyingly counterintuitive as Zwicky's: either our laws of physics don't work at galactic scales, or there's a dim and/or invisible component to their recipe. What is nature trying to tell us? New physics or dark matter? With only the results of Vera and Fritz (now that's a sitcom) to work from, we can't tell the difference between the alternatives.

We've been in this situation before, with a set of observations contradicting what we expect. It's kind of how science advances, so at least this is familiar territory. And in the case of gravity, we've taken both paths in the past. For example, Mercury. Tiny little Mercury, the closest to the sun and the most swift-footed of the planets. It has the most elliptical orbit of all the planets (not counting the dwarfs like Pluto and Eris), and the point where Mercury comes closest to our sun lazily traces out a circle over the years. Most of this motion is due to the gentle but persistent gravitational tugs of the outer planets, but a small part of that motion couldn't be explained by the gravity of Newton.

Perhaps it was another planet in our solar system, an inner-inner world of fire, nicknamed Vulcan, orbiting close enough to the sun to remain hidden to the ancients, only making its presence known by gravitational flirtations with Mercury. But searches turned up empty. The answer here was new physics: one of the first clues Einstein had that he was on the right track was that general relativity could fully explain Mercury's orbital oddities.3 In other words, the portrait of gravity as painted by Newton breaks down close to the sun, and it takes a revolutionary new view of the cosmos to understand what's going on.

So maybe the quandary presented by Rubin and Zwicky could be explained by new physics. Maybe general relativity, just like Newtonian physics before it, can't cut the mustard past a certain scale. We love you, Albert, and your theory is a thing of beauty. But maybe it's just not good enough, pal.

But to be perfectly honest, the Vulcan approach has worked in the past. The orbit of Uranus was also behaving oddly, given the known denizens of the solar system and our Newtonian knowledge of gravity. Instead of modifying Newton to account for the observations, astronomers instead posited the existence of a new planet to explain the curious orbits in the outer solar system, and in due time the agent provocateur was found—the planet Neptune.⁴

In that case, it wasn't new physics but previously unknown matter that best explained observations.

But what to do with galaxies and clusters? Well, when nature fights dirty, fight back—with science.

In the decades since Rubin's reinvigoration of the dark matter debate, astronomers around the world have embarked on an all-out observational war, measuring, comparing, and testing galaxy after galaxy and cluster after cluster with as many methods as humanly (and, in the age of more advanced technology, robotically) possible.

I'm going to be polite here and warn you that I'm about to absolutely inundate you with more evidence for the existence of dark matter. It's not that I'm trying to beat this concept into your brain, but—no, wait, that's exactly what I'm trying to do. The reason is that the mystery of dark matter persists to the present day (at least, to the time of me writing this book). We have tons of rock-solid evidence that something funny is going on out there in the great expanse, but we're much more hazy on what's causing it.

Because of this, there's a lot of fear, uncertainty, and doubt among the general public (surely not you, but maybe someone you know) when it comes to understanding this facet of our universe. In the past few centuries, we've solved a lot of mysteries of the heavens above us, but continued observations have revealed deeper, perhaps more sinister machinations in the motions of celestial objects. One of them is the nature of the earliest moments of the big bang, when forces were so extreme and exotic that we have trouble even theoretically navigating them.

The other is here, in the old and cold universe of today. It's not some relic of the distant past, a problem that we can leave on the doorstep of future generations of scientists, safe in the knowledge that our overall picture is coherent. Dark matter is present today, in galaxies and clusters all around you. And when I get there, you'll see that it's probably inside you too.

We've already seen Zwicky's realization that dunkle Materie might be a thing inside clusters of galaxies based on the motions of galaxies whizzing around inside them. But threaded between those galaxies is an incredibly hot (up to a hundred million Kelvin—that's hotter than the core of the sun) but incredibly thin (about one thousand particles per cubic meter; compare that to the 10²⁵ air molecules per cubic meter that you're breathing right now) plasma. It's so thin that the particles—protons and electrons—travel for about a light-year before interacting with each other, but when they do, they emit X-ray radiation in a process known as Bremsstrahlung. It's German for “braking radiation,” and it may or may not be one of my favorite words in physics.5

So the gas is hot and emits X-rays. The hotter it is, the more X-rays come out; ergo, we can measure the intensity of X-rays and estimate the gas temperature. And just as with the galaxies themselves, there's a connection between the temperature of the gas and the total mass of the cluster. If the cluster is in equilibrium, then the gas can't be too hot (or the cluster explodes) or too cold (or it collapses). Since that is obviously not happening, we can get a rough handle on cluster masses. Result: not all the cluster mass is visible.

Here's another one. Let's rewind all the way back to the first dozen minutes of the universe, when all the protons and neutrons were hitching up to form the universe's initial supply of hydrogen, helium, and lithium. The prediction of the abundance of those light elements, spawned from our understanding of nuclear physics, was (and is) a triumph of the power of the big bang model. Those same calculations place a hard upper limit on the total amount of baryonic (if you remember your jargon lessons, that's a particle like a proton or neutron) material available in the universe. When we compare that to all our cluster observations, we end up with a number that's about one-fifth too shy. So in principle, we're measuring the same thing, the mass of the universe, but at different times (in the first minutes versus the latest billion years). Those numbers ought to be the same, because where is all the mass going to go? But instead the clusters are much fatter than they should be, given the elemental building blocks available to them.

Check this one out. Remember that story of inflation and the early gravitational growth of structures? It's a good story and worth remembering. There's one big caveat that I deliberately neglected to mention, because I wanted to save it for this moment: in order for the galaxies to have the sizes they do, we need a form of dark matter. Specifically, a kind of dark matter that doesn't interact with light. The problem with pure baryons is that they get easily distracted. Gravity tries to pull them together, but the intense radiation pulls them apart, a process we saw play out with agonizing repetition until matter and radiation finally went their separate ways with the birth of the cosmic microwave background.

But by the time of recombination, a few hundred thousand years into the history of the universe, the seeds were already planted…by dark matter. Some form of invisible matter could freely ignore radiation, pooling itself together in those early years, creating the nests that baryons would eventually collect in to start building larger structures. In other words, even though we can't see it, the cosmic microwave background and the later structures to emerge depends on the presence of dark matter.

Given only baryonic processes operating in the early universe, there simply wasn't enough time to build galaxies, including the Milky Way. Including you. Yes, you. Without dark matter, high-density structures (and I'm not calling you dense, per se, just noting your density relative to—never mind) couldn't have formed.

Not convinced yet? Let's take a look at gravitational lensing. Matter, whatever its form, will bend space-time, which will deflect that path of light. We've tested this like crazy. So if you're looking at, say, a gigantic galaxy cluster and wondering about its mass, you can use the bent light from background objects to figure it out. If you look at a distorted image through a regular glass lens, and you know what shape the undistorted image ought to have, you can use your Knowledge of Physics to figure out the properties of the lens doing the distortion.

We know what galaxies look like because we have, well, a lot of samples. So when light from a distant galaxy passes through a not-so-distant cluster, that image gets distorted, and we can compare it to what galaxies look like without a funhouse mirror and Sherlock out the properties of the cluster doing the lensing. You know, its mass.

So that's a completely totally different method for measuring the mass of big cosmological objects. And guess what answer it gives?

Yup: large objects in the universe are more massive than they appear at first glance.

I don't know about you, but I'm getting the feeling that nature is trying to tell us something.

The evidence has been piling up for decades now, and our options for explaining that evidence—what nature is practically shoving in our faces every time we go to ask the very simple question of how much a galaxy or cluster weighs—have severely narrowed in the time since Rubin's, and especially Zwicky's, initial results.

Can all these combined results—galaxy rotation curves, cluster masses, the cosmic microwave background, early-universe physics, the mere existence of galaxies, and gravitational lensing—be explained by new physics? Is Einstein not enough? It's always a possibility, but as the years go by, it becomes increasingly slim.

The first serious attempt came shortly after Rubin's observations with a theory called MOND (once again, I'm not in charge of naming things). Short for Modified Newtonian Dynamics, it's exactly what it says it is—it modified Newton's fundamental laws so that the relationship of mass, acceleration, and force isn't what we're used to.6 The key is that everything appears perfectly normal on the surface of the Earth or in the solar system, but once you get to galactic scales, it breaks down and has to be, well, modified.

This works great for explaining galaxy rotation curves, because it was explicitly designed to work great for explaining galaxy rotation curves. No invisible matter here, just different physics at work. But in order to make a theory capable of coherently explaining both galactic and early-universe observations, it needed to be elevated to be fully relativistic—in other words, it had to be written in a way similar to special relativity (which, dark matter or not, nobody was arguing against). That's the game we have to play in cosmology: if you're not relativistic, you don't have enough explanatory oomph to take you everywhere you need to go.

The result of marrying MOND with relativity is called TeVeS, for Tensor-Vector-Scalar. I won't go into the messy details, but the short version is that it's largely ruled out: TeVeS predicts certain results for gravitational lensing and the cosmic microwave background that don't agree with observations.7

The real bullet that finally killed modified laws of physics was detailed observations of the appropriately named Bullet Cluster.⁸ That cluster of galaxies is itself a train wreck (though not to be confused with the Train Wreck Cluster, which is something else), where two massive clusters slammed into each other long ago, and we have a pretty picture of the wreckage. When we look at this system with different techniques, like a team of forensic investigators trying to understand a murder, we get a complete picture of what's going on.

First, the galaxies. Galaxies are like buzzing little bees in the giant volume of their host clusters. When clusters collide, it's like two swarms of bees headed in the same direction: for the most part, they just sail on through without interacting. So the galaxies end up on opposite sides of where they started. Fine, nothing surprising there.

Next, the hot, thin gas between the galaxies that fills up the bulk of the cluster. As thin as it is, it still can't help but get tangled up with its counterpart in the opposite cluster during the merger event. And when those two giant balls of gas slam into each other, we get all the rich and glorious physical interactions that we've come to expect with interacting balls of hot gas: cold fronts, shock waves, instabilities, the works. When we examine the Bullet Cluster with X-rays, we see all the fireworks happening in the center of the interaction, with the two sets of galaxies sitting on opposite sides, safely navigating the merger event unscathed.

Now, where's the mass? We have exquisitely good gravitational lensing maps of the Bullet Cluster, showing not only how much total material the colliding pair hosts but also where it's distributed in space. How handy is that? Those lensing maps reveal a curious pattern: the concentration of mass is not tangled up with the hot gas in the center but is more closely associated with the galaxies. Even then, though, it's not mapped directly to the galaxies themselves but, rather, smoothly distributed throughout the remnants of the clusters.

That hidden mass is much, much larger than can be provided by the galaxies alone.

There's no picture of modified gravity that can sufficiently explain what we see with the Bullet combined with every other observation we have of the universe. I know, I know. It would have been super awesome to have a handle to lever ourselves up and past Einstein's relativity. A couple of Nobel Prizes would have been tossed around, and we'd be working on the Next Big Challenge.

Oh well. Like I said, nature isn't playing fair.

So Einstein gets to stay in the game, but as I said earlier, something has to give. The universe must have some additional, previously unknown component.

Maybe it's normal stuff, just dim. Like dead or failed stars, rogue planets, or black holes. Those don't give off a lot of light (obviously), and with a bit of finagling and just the right circulation, they would have all the right properties for galaxy rotation curves, lensing, and all the rest. But the only way to make black holes is through the death of massive stars. And the only way to make botched stars is to have giant clouds of star-forming material that failed to ignite fusion. To have the dark matter really be dim but otherwise normal matter, we need a lot of…normal matter, and that's ruled out by our knowledge of the early universe, like the processes that build the first nuclei and the growth of the first galaxies.9

There are, of course, a bunch of half-baked, and sometimes quarter-baked, ideas floating around in academic circles, and it would be exhausting to give an exhaustive review. Give me five theorists, and I'll walk away with six theories on dark matter. I wanted to insert this caveat because of a profound and—dare I say—noble sense of completeness, but I'm not going to spend a lot of time on them because they're usually pretty dang awkward solutions to the dark matter problem, and not as fleshed out and agreeably comprehensive as the one I'm about to present.

Which is WIMPs. That's right: WIMPs. Astronomers, as we've seen, have a flair for the ridiculous acronym. Weakly Interacting Massive Particles. The main historical competitors to WIMPs were the MACHOs—the MAssive Compact Halo Object, the name given to the chunks of “normal” matter, like failed stars, that might have explained the observations but never quite did.

So our best solution to the dark matter problem is called a WIMP, and we're just going to have to live with that.

To explain what the heck a WIMP is and why you should care, I need to mention one other property that dark matter, whatever it is, has to have: it has to be cold. That means that at the time it comes on the scene in a big way, it has to have speeds much lower than that of light. The reason is structure. If the invisible part of our universe is too “hot,” then it has a much easier time ignoring the effects of gravity, and this “washes out” smaller structures like galaxies. Since galaxies very much exist, the dark matter has to help rather than hurt the formation of structure billions of years ago, which means it has to be amenable to the flirtatious whisperings of gravity.10

Thus particles like the neutrino, which absolutely inundate the universe and (this turned out to be a surprise) have a little bit of mass, aren't a good candidate to be the dark matter—they're too hot, and if you added too many of them into the recipe of the universe, enough to point to it and say, “That's the dark matter—we've known it all along!,” the resulting pastry will be too bland and smooth, lacking the delicious, flaky layers of our modern cosmos.

I've been dancing around this issue up until now, so I'll get right to it: as best as we can tell, the explanation for the dark matter problem in our universe is a new kind of subatomic particle previously unknown to science. Something that doesn't interact with light, probably doesn't even interact with itself, and is cold.

And our best theoretical candidates for that particle is the WIMP, a (P)article that is (M)assive and (I)nteracts only via the (W)eak nuclear force. Why this one? Well, cosmologists aren't the only people on the planet hunting for never-before-seen particles. Those crafty high-energy theorists are constantly churning out idea after idea, coming up with ways to break the chains of the standard model of particle physics and extend our knowledge of the subatomic realm. The initial forays into the uncharted territory of new physics predict the existence of a host of new particles, just as Dirac's musings led us on the path to antimatter.

These predicted particles just so happen, by sheer happy coincidence, to have the right properties to explain the dark matter as we see it in the universe, and especially to have the right properties in the early universe to form the seeds of the kinds of structures that we observe at large scales today.¹¹

Of course there are many untested routes beyond the standard model, and these routes differ in their predictions of what precisely the dark matter might be—if there is even a single particle responsible for all that mass, and not a family. But the good news of WIMPs is that these hypotheses make testable predictions, so we can at least try to rule them out.

If dark matter is truly a WIMP, then each and every galaxy is flooded with them. The same way you are constantly surrounded and bombarded by radiation, which you notice, and neutrinos, which you don't, trillions upon countless trillions of WIMP particles are zipping through your room right now, through the hands holding this book and through the brain thinking these thoughts. You're drowning in dark matter.

You don't notice because—as per the definition—it only interacts via the weak force, which is very rare. We don't notice the effects of its gravity on small scales like the solar system because it's relatively smooth. Gravity only cares about differences in density, so you have to get out to supergalactic scales to notice any substantial variations and the interesting gravitational properties that come into play.

Right now, as I type and probably as you read, there are detectors all around the world dedicated to hunting for a dark matter particle, hoping to catch enough rare chance interactions to confirm a detection of the dark matter particle. So far, they've turned up empty, which has helped rule out some models and tighten the noose on the exact properties of the WIMP.

While they're still the most appealing candidate for the dark matter, the attitude toward WIMPs has taken a more skeptical bent as of late, especially after the Large Hadron Collider smashed some particles together to look for paths leading away from the standard model. Unfortunately, those searches have—so far—turned up empty, and the simplest and easiest paths, some of them containing a WIMP candidate, appear to be dead ends.

That said, there are a few other candidate ideas floating around the journals and blackboards of academia that predict new particles without them being strictly WIMPy, so there are still a lot of uncovered stones.

Again bowing to that noble sense of completeness, I need to mention that dark matter as an explanation for our cosmic observation does have some weaknesses, dealing with the detailed structures of galaxies and the numbers of satellites a galaxy might have. Those are, of course, topics of active research and vigorous debate.12

Disclaimers out of the way, this is how we do science. You pick the explanation that has the fewest assumptions and is able to explain the most observations. It's a very simple strategy that has uncovered and unlocked mystery after mystery in the cosmos. And despite our best efforts to develop a viable alternative, it appears that we live in a universe dominated by an exotic, cold, invisible form of matter.

This particle, if it exists, lives outside the standard model. Nothing we know of—not the neutrino, not any of the quarks or mesons or whatever-ons—can explain the cosmic observations. Something funny is going on in the universe at large scales, and that funny something is deeply related to our unquenchable thirst to understand physics at its deepest level.

Whatever that particle is, whatever its true nature and intent, it is by a wide margin the dominant player of the matter game in our universe, beating our familiar light-loving baryons at least five to one. So when you enjoy a clear dark night dotted with countless sparkling stars, you're looking at the mere representatives of cosmic structures. All the particles you know and love, the ones that constitute the physics of the familiar, of the warmth of sunlight on a summer's day, of the solid rocks underneath your feet, of the exchange of ions in your blood vessels, are a minority.

Stars and even galaxies are lighthouses on a distant, hidden shore. A beacon of light, signaling the presence of larger masses, tracing their outlines without revealing more. The journey started by Zwicky and Rubin, and continuing to the present day, leads us to an inescapable and uncomfortable conclusion: we do indeed live in a dark, cold universe.