You can’t get mad at a team for trying the improbable, hoping that nature cooperates. Some of the most famous discoveries of all time have come about thanks to nothing more than mere serendipity, and so if we can test something at low-cost with an insanely high reward, we tend to go for it. Believe it or not, that’s the mindset that’s driving the direct searches for dark matter.

In order to understand how to find dark matter, however, you have to first understand what we know so far, and what the evidence points to as far as direct detection goes. We haven’t found it yet, but that’s okay. Not finding dark matter in an experiment is not evidence that dark matter doesn’t exist. The indirect evidence all shows that it’s real. The question before us is how to demonstrate its reality, hopefully by finding the particle responsible for it directly.

The particles and antiparticles of the Standard Model of particle physics are exactly in line with what experiments require, with only massive neutrinos providing a difficulty and requiring beyond-the-standard-model physics. Dark matter, whatever it is, cannot be any one of these particles, nor can it be a composite of these particles. (E. SIEGEL / BEYOND THE GALAXY)

Let’s begin with a basic recap of dark matter: the idea, the motivation, the observations, the theory and then we’ll talk about the hunt.

The idea. You know the basics: there are all the protons, neutrons and electrons that make up our bodies, our planet and all the matter we’re familiar with, as well as some photons (light, radiation, etc.) thrown in there for good measure. Protons and neutrons can be broken up into even more fundamental particles — the quarks and gluons — and along with the other Standard Model particles, make up all the known matter in the Universe.

The big idea of dark matter is that there’s something other than these known particles contributing in a significant way to the total amounts of matter in the Universe. Why would we think such a thing?

The two bright, large galaxies at the center of the Coma Cluster, NGC 4889 (left) and the slightly smaller NGC 4874 (right), each exceed a million light years in size. But the galaxies on the outskirts, zipping around so rapidly, points to the existence of a large halo of dark matter throughout the entire cluster. (ADAM BLOCK/MOUNT LEMMON SKYCENTER/UNIVERSITY OF ARIZONA)

The motivation. We know how stars work, and we know how gravity works. If we look at galaxies, clusters of galaxies and go all the way up to the largest-scale structures in the Universe, we can extrapolate two things. One: how much mass there is in these structures at every level. We look at the motions of these objects, we look at the gravitational rules that govern orbiting bodies, whether something is bound or not, how it rotates, how structure forms, etc., and we get a number for how much matter there has to be in there. Two: we know how stars work, so as long as we can measure the starlight coming from these objects, we can know how much mass is there in stars.

These two numbers don’t match, and they don’t match spectacularly. There had to be something more than just stars responsible for the vast majority of mass in the Universe. This is true for the stars within individual galaxies of all sizes all the way up to the largest clusters of thousands of galaxies in the Universe.

The predicted abundances of helium-4, deuterium, helium-3 and lithium-7 as predicted by Big Bang Nucleosynthesis, with observations shown in the red circles. The Universe is 75–76% hydrogen, 24–25% helium, a little bit of deuterium and helium-3, and a trace amount of lithium by mass. After tritium and beryllium decay away, this is what we’re left with, and this remains unchanged until stars form. Only about 1/6th of the Universe’s matter can be in the form of this normal (baryonic, or atom-like) matter. (NASA / WMAP SCIENCE TEAM)

The observations. This is where it gets fun, because there are a ton of them; I’ll focus on just three. When we extrapolate the laws of physics all the way back to the earliest times in the Universe, we find that there was not only a time so early when the Universe was hot enough that neutral atoms couldn’t form, but there was a time where even nuclei couldn’t form! The formation of the first elements in the Universe after the Big Bang — due to Big Bang Nucleosynthesis — tells us with very, very small errors how much total “normal matter” is there in the Universe. Although there is significantly more than what’s around in stars, it’s only about one-sixth of the total amount of matter we know is there.

The fluctuations in the Cosmic Microwave Background were first measured accurately by COBE in the 1990s, then more accurately by WMAP in the 2000s and Planck (above) in the 2010s. This image encodes a huge amount of information about the early Universe, including its composition, age, and history. The fluctuations are only tens to hundreds of microkelvin in magnitude, but definitively point to the existence of both normal and dark matter in a 1:5 ratio. (ESA AND THE PLANCK COLLABORATION)

The fluctuations in the cosmic microwave background are particularly interesting. They tell us what fraction of the Universe is in the form of normal (protons+neutrons+electrons) matter, what fraction is in radiation, and what fraction is in non-normal, or dark matter, among other things. Again, they give us that same ratio: that dark matter is about five-sixths of all the matter in the Universe.

The observations of baryon acoustic oscillations in the magnitude where they’re seen, on large scales, indicate that the Universe is made of mostly dark matter, with only a small percentage of normal matter causing these ‘wiggles’ in the graph above. (MICHAEL KUHLEN, MARK VOGELSBERGER, AND RAUL ANGULO)

And finally, there’s how structure forms on the largest scales. This is particularly important, because we can not only see the ratio of normal-to-dark matter in the magnitude of the wiggles in the graph above, but we can tell that the dark matter is cold, or moving below a certain speed even when the Universe is very young. This pieces of knowledge lead to outstanding, precise theoretical predictions.

According to models and simulations, all galaxies should be embedded in dark matter halos, whose densities peak at the galactic centers. On long enough timescales, of perhaps a billion years, a single dark matter particle from the outskirts of the halo will complete one orbit. The effects of gas, feedback, star formation, supernovae, and radiation all complicate this environment, making it extremely difficult to extract universal dark matter predictions.(NASA, ESA, AND T. BROWN AND J. TUMLINSON (STSCI))

The theory. This tells us that around every galaxy and cluster of galaxies, there should be an extremely large, diffuse halo of dark matter. This dark matter should have practically no “collisions” with normal matter — upper limits indicate that it would take light-years of solid lead for a dark matter particle to have a 50/50 shot of interacting just once — there should be plenty of dark matter particles passing undetected through Earth, me and you every second, and dark matter should also not collide or interact with itself, the way normal matter does.

There are some indirect ways of detecting this: the first is to study what’s called gravitational lensing.

When there are bright, massive galaxies in the background of a cluster, their light will get stretched, magnified and distorted due to the general relativistic effects known as gravitational lensing. (NASA, ESA, AND JOHAN RICHARD (CALTECH, USA) ACKNOWLEDGEMENT: DAVIDE DE MARTIN & JAMES LONG (ESA / HUBBLE)NASA, ESA, AND J. LOTZ AND THE HFF TEAM, STSCI)

By looking at how the background light gets distorted by the presence of intervening mass (solely from the laws of general relativity), we can reconstruct how much mass is in that object. There’s got to be dark matter in there, but from looking at colliding clusters of galaxies, we learn something even more profound.

The gravitational lensing map (blue), overlayed over the optical and X-ray (pink) data of the Bullet cluster. The mismatch of the locations of the X-rays and the inferred mass is undeniable. (X-RAY: NASA/CXC/CFA/M.MARKEVITCH ET AL.; LENSING MAP: NASA/STSCI; ESO WFI; MAGELLAN/U.ARIZONA/D.CLOWE ET AL.; OPTICAL: NASA/STSCI; MAGELLAN/U.ARIZONA/D.CLOWE ET AL.)

The dark matter really does pass right through one another, and accounts for the vast majority of the mass; the normal matter in the form of gas creates shocks (in X-ray/pink, above), and only accounts for some 15% of the total mass in there. In other words, about five-sixths of that mass is dark matter! By looking at colliding galaxy clusters and monitoring how both the observable matter and the total gravitational mass behaves, we can come up with an astrophysical, empirical proof for the existence of dark matter.

But that’s indirect; we know there’s supposed to be a particle associated with it, and that’s what the hunt is all about.

If dark matter does have a self-interaction, its cross-section is tremendously low, as direct detection experiments have shown. It also doesn’t scatter very much off of nuclei. (Mirabolfathi, Nader arXiv:1308.0044 [astro-ph.IM])

The hunt. This is the great hope: for direct detection. Because we don’t know what’s beyond the standard model — we’ve never discovered a single particle not encompassed by it — we don’t know what dark matter’s particle (or particles) properties should be, should look like, or how to find it. We don’t even know if it’s all one thing, or if it’s made up of a variety of different particles.

So we look at what we’d be able to detect instead, and look there. We can look for interactions down to a certain cross-section, but no lower. We can look for energy recoils down to a certain minimum energy, but no lower. And at some point, experimental limitations — natural radioactivity, cosmic neutrons, solar/cosmic neutrinos, etc. — make it impossible to extract a signal below a certain threshold.

Hall B of LNGS with XENON installations, with the detector installed inside the large water shield. If there’s any non-zero cross section between dark matter and normal matter, not only will an experiment like this have a chance at detecting dark matter directly, but there’s a chance that dark matter will eventually interact with your human body. (INFN)

Long story short: the latest experiment to search for dark matter directly didn’t find it, at least not yet. That’s been the story for every direct detection experiment ever performed, confirmed, and robustly tested, over and over again.

And that’s okay! Unless dark matter happens to be of a certain mass with a certain interaction cross-section, none of the designed experiments are going to see it. That doesn’t mean dark matter isn’t real, it just means that dark matter is something else than what our experiments are optimized to find.

The cryogenic setup of one of the experiments looking to exploit the hypothetical interactions between dark matter and electromagnetism. Yet if dark matter doesn’t have specific properties that current experiments are testing for, none of the ones we’ve even imagined will ever see it directly. (AXION DARK MATTER EXPERIMENT (ADMX) / LLNL’S FLICKR)

So we keep looking, we keep thinking of new possibilities for what it could be, and we keep thinking of new ways to search for it. That’s what science at the frontiers is like. Personally, I don’t expect these direct detection attempts to be successful; we’re stabbing in the dark hoping we hit something, and there are little-to-no good reasons for dark matter to be in these ranges. But it’s what we could see, so we go for it. If we find it, Nobel Prizes and new physics discoveries for everyone, and if we don’t, we know a little more about where the new physics isn’t. But just as you shouldn’t fall for the hyper-sensationalized claims that dark matter has been directly detected, you shouldn’t fall for the ones that say “there’s no dark matter” because a direct detection experiment failed.

We are after the most fundamental stuff in the Universe, and we’ve only recently begun to understand it. It shouldn’t be a surprise if the search takes a little — or even a lot — longer. In the meantime, the journey for knowledge and understanding of just what it is that holds the Universe together continues.