Inside the search for dark matter: Leading experts at SLAC share their journey and prospects for finding the missing stuff of our universe

Over a decade ago, dark matter experts Daniel Akerib and Thomas Shutt joined the Department of Energy’s SLAC National Accelerator Laboratory, continuing their mission to uncover the elusive substance. We recently caught up with them to discuss the current state of the dark matter search.

What is dark matter and why are we trying so hard to find it?

Dan: Dark matter is an invisible substance that makes up 85% of the matter of the universe, yet we don’t know exactly what it is. Without dark matter, galaxies—and therefore life – won’t yet have formed. That’s why studying it is so important. It’s not just about solving a puzzle – it’s about understanding the nature of our existence.

This missing mass isn't just theoretical – it’s visible through the gravitational effects it has on nearby galaxies and observed in patterns in the cosmic microwave background, the faint glow left over from the Big Bang.

On the scale of galaxies and larger structures, we either need to rewrite the laws of gravity or we need to find the missing stuff that’s producing the gravitational effects we clearly observe. We don't know how to get from the Big Bang to the formation of galaxies without dark matter.

Tom: Physics is all about exploring profound questions. The mathematical framework we’ve developed explains much of the universe’s evolution, from the Big Bang to today. But the dominant form of mass in the universe – that so far we have observed only through its gravitational effects – isn’t normal matter. This gap in our understanding is both humbling and exciting. How can we claim to understand the universe when such a significant piece is missing? Exploring that mystery drives us forward.

How are we looking for dark matter?

Dan: Currently, there’s no convincing theory to replace Newton’s or Einstein’s understanding of gravity. Instead, scientists have developed hypotheses about what this missing “stuff” could be, and we’ve been testing these ideas for decades.

To learn more, scientists are trying to create dark matter in laboratories by simulating conditions similar to the Big Bang. For example, smashing protons together might produce particles related to dark matter. These experiments help link cosmic phenomena with what we can test in the lab.

Tom: Another approach is direct detection. Scientists build highly sensitive detectors to catch signals from potential interactions between dark matter particles and normal particles – like a tiny billiard-ball collision. These detectors aim to provide definitive evidence of dark matter’s existence and properties. This is the approach Dan and I have been collaborating on for decades.

What are the challenges in discovering dark matter, specifically through direct detection?

Dan: Every experiment faces interference, and direct detection of dark matter is no exception. The challenge is identifying the signal we want to detect while eliminating anything that could mask or mimic it.

Think of it like this: The stars are always shining, even during the day, but we can’t see them because of sunlight scattered in the atmosphere that makes the blue sky. That’s why we observe stars at night. Similarly, our instruments must contend with various sources of background interference that can obscure the dark matter signal.

One major challenge is cosmic rays. If we conduct these experiments on the Earth’s surface, cosmic rays bombard our instruments and overwhelm the signal. To address this, we need to place our detectors deep underground and in shielded environments to reduce this noise.

Tom: Radioactivity is another obstacle. Even tiny amounts of radiation can mimic the signals we’re trying to detect. To overcome this, we’ve had to become experts at eliminating all traces of radioactivity – far beyond what most people might imagine. The core environment of our detectors must be up to a trillion times less radioactive than our everyday surroundings. This level of precision is necessary because even the faintest radioactive signal could interfere with our ability to detect dark matter.

We’re still not sure what dark matter is. Can you tell us about what we have learned so far from previous dark matter searches?

Tom: The two strongest candidates for dark matter have long been WIMPs (Weakly Interacting Massive Particles) and axions. 

In the mid-1990s, researchers proposed using liquid xenon as a detection material, and by the 2000s, it emerged as the most promising new technology. The first major success came around 2007, and since then, liquid xenon detectors have been central to every major dark matter result, with increasing sensitivity as we scaled up detector size and refined methods.

Dan: Xenon detectors will either close this chapter of the dark matter story – by detecting it or by reaching a fundamental limit imposed by nature, something we call the neutrino fog.

The neutrino fog is an inherent challenge unrelated to dark matter itself. Eventually, neutrinos will appear in our detectors at such a level that we won’t be able to distinguish them from potential dark matter signals. It’s a natural limit to what these instruments can achieve.

It’s kind of bittersweet to think about – hitting that wall where nature itself prevents us from seeing any further.

So, you're saying we’ll hit a point where we won’t be able to detect anything further?

Tom: Exactly. At some point, we’ll reach a practical limit where it will be challenging to detect anything further because the signals will be buried in the neutrino fog. And the hard truth is, no one will know for sure if we just missed detecting dark matter – it could just be hiding right below that fog.

Getting past that limit would require developments far beyond anything we currently have in hand. While it’s not theoretically impossible, the cost and complexity would be so enormous that, in practical terms, it’s game over once we hit that point, at least until we learn more about solar neutrinos and develop new methods and technology.

Dan: And honestly, we’re inching closer to that limit now. Speaking as two guys in our 60s, it’s humbling to think about!

So with liquid xenon, once you run an experiment and don’t see anything, what do you do next to improve the design? What are the next steps?

Tom: Bigger and cleaner!

Dan: That’s always the approach. The LUX-ZEPLIN (LZ) experiment, for instance, has an inner detector that is about 1.5 meters in diameter and 1.5 meters tall. For our future project, we’re envisioning a detector that’s 3 meters in diameter and 3 to 4 meters tall – holding around 60 to 80 tons of active xenon. That’s ten times the amount we’re currently working with.

Tom: Scaling up like this requires meticulous improvements. You’re not just building a bigger detector – you’re also making it cleaner by further reducing radioactivity and background noise. Every component of the detector needs to be redesigned to be ten times cleaner. This combination of increased volume and decreased interference allows us to push the limits of what we can detect.

What role did Snowmass and the Particle Physics Project Prioritization Panel (P5) play in shaping the future of dark matter experiments?

Dan: Snowmass is a community-driven process to explore the future of high-energy physics, and as part of LZ, we pitched the "Generation 3" (G3) dark matter experiment. It’s a big step forward, aiming to partner with European projects to push the technology to the ultimate background limit.

In summer 2022, just before Snowmass, LZ released its first results, which made a strong impression at the conference. The consensus was clear: If WIMPs exist in our target range, we need to see this through now. 

The Snowmass discussions led to a recommendation from P5 that DOE support a Generation 3 dark matter experiment. 

You've each been searching for dark matter for three decades. What do you find most exciting about this Generation 3 search? 

Dan: Someone recently asked me about “opening the box” – that moment when you analyze the data, hoping to find a signal. I’ve opened that box about a dozen times now, and each time, it’s led to a world-best result. Even though we didn’t discover dark matter, each of these results has put the most stringent limits yet on what it could be. The thrill remains the same: With each new experiment, there’s always a chance of discovery.

Tom: That’s true, and the story of dark matter is still compelling, even though we haven’t detected it in the ways theorists originally imagined. It’s like trying to kick a football that keeps moving. 

As finding WIMPs and axions proved more difficult, theorists began exploring alternative possibilities. They asked, “What else could fit within the framework of particle physics that hasn’t been ruled out?” Philip Schuster and Natalia Toro led this thinking, proposing new dark sector particles. This inspired projects like the proposed Light Dark Matter Experiment (LDMX).

Meanwhile, Kent Irwin and Peter Graham explored innovative ways, such as the Dark Matter Radio, to detect axion-like particles using superconducting sensors. The rise of quantum computing has made these experiments more feasible, and axion searches are advancing rapidly.

Dan: We’re leading multiple efforts at the lab to improve dark matter searches and explore a range of possibilities. It’s an expanded search portfolio – attacking the problem from all angles. As we keep advancing, dark matter will have an increasingly hard time hiding from SLAC.

For questions or comments, contact SLAC Strategic Communications & External Affairs at communications@slac.stanford.edu.