85% of the matter in the world is missing. But we are getting closer to finding it

Most of the matter in the universe is missing. Scientists believe that about 85% of the matter in the universe is made up of invisible dark matter, which is only indirectly detected by its gravitational effects on its surroundings.

My colleagues and I — a team of 250 scientists from around the world working on the dark matter experiment called LUX-ZEPLIN (or LZ) — today report our latest findings from a long search to discover exactly what it’s made of.

We still haven’t found the elusive particles that we believe dark matter is made of, but we’ve set the tightest limits yet on their properties. We have also shown that our detector works as expected – and should produce better results in the future.

Our results were reported today at the TeV Particle Astrophysics 2024 conference in Chicago and the LIDINE 2024 conference in São Paulo, Brazil. The journal article will be submitted for peer review in the coming weeks.

What is dark matter?

When astronomers look at the universe, they see evidence that the visible matter of stars, gas, and galaxies is not everything. Many phenomena, such as the speed of rotation of galaxies and the pattern of the glow left over from the Big Bang, can only be explained by the presence of large amounts of invisible matter – dark matter.

So what is this dark matter made of? We currently do not know of any type of particle that could explain these astronomical observations.

There are dozens of theories that aim to explain observations of dark matter, from strange unknown particles to tiny black holes or fundamental changes in our theory of gravity. However, none of them have been proven correct yet.

One of the most popular theories suggests that dark matter is made up of so-called “weakly interacting massive particles” (or WIMPs). These relatively heavy particles can cause the observed gravitational effects and also—rarely—interact with ordinary matter.

How do we know that this theory is true? Well, we think that these particles must always flow through the earth. For the most part, they pass by without interacting with anything, but every once in a while a WIMP may hit the nucleus of an atom directly—and these collisions are what we’re trying to detect.

A large cold tank of liquid xenon

The LZ experiment is located in an old gold mine about 1,500 meters underground in South Dakota in the United States. Placing the experiment deep in the basement helps cut off background radiation as much as possible.

The experiment consists of a large double-walled tank filled with seven tons of liquid xenon, a noble gas, cooled to a temperature of 175 Kelvin (-98 degrees Celsius).

If a dark matter particle hits a xenon nucleus, it should emit a small flash of light. Our detector has 494 light sensors to detect these flashes.

Of course, dark matter particles are not the only things that can cause these flashes. There is still some background radiation from the surroundings and even the tank material and the detectors themselves.

A big part of figuring out whether we’re seeing signs of dark matter is separating this background radiation from anything weirder. To do this, we perform detailed simulations of the results we would expect to see with and without dark matter.

These simulations have been the focus of a large part of my experimental department since I started my PhD in 2015. 2021.

Pull the net tighter

Our latest results show no sign of dark matter. However, they allowed us to rule out many possibilities.

We found no traces of particles with masses greater than 1.6 × 10^-26 kg, which is about ten times heavier than a proton.

These results are based on 280 days of observations from the detector. Ultimately, we aim to collect 1,000 days worth, which will allow us to search for more potential dark matter particles.

If we’re lucky, we might find dark matter in the new data. If not, we have already started planning the next generation dark matter experiment. The XLZD (XENON-LUX-ZEPLIN-DARWIN) consortium plans to build a detector nearly ten times larger, allowing us to look even further into space where these ubiquitous yet elusive particles are hidden.