Deep beneath a mountain in Italy, far from sunlight and much of the radiation that constantly reaches Earth’s surface, physicists have spent decades searching for one of the universe’s greatest mysteries: dark matter.
Their instruments are designed to detect extraordinarily rare events. Most particles pass through the surrounding rock without leaving a trace. If a dark-matter particle occasionally collided with an atom inside a detector, however, it might produce a tiny flash of light or another measurable signal.
One experiment, known as DAMA/LIBRA, has reported a recurring pattern in its measurements for more than two decades. The signal appears to rise and fall over the course of a year in a way that the research team interprets as consistent with the expected movement of Earth through the Milky Way’s dark-matter halo.
But there is a problem.
Other experiments have searched for a comparable signal and failed to reproduce the result—even when using the same detector material.
The disagreement has become one of the most persistent puzzles in experimental dark-matter physics. It raises a fundamental question: is DAMA/LIBRA detecting an elusive substance that has escaped other experiments, or is something else responsible for the pattern?
The answer matters because dark matter appears to account for a substantial fraction of the universe’s matter. Yet scientists still do not know what it is made of.
The underground laboratory built to find the invisible
The DAMA/LIBRA experiment operates at the Gran Sasso National Laboratory in Italy, beneath a thick layer of mountain rock.
That location is not incidental. Earth’s atmosphere is constantly struck by high-energy particles from space called cosmic rays. Their interactions can produce showers of secondary particles that interfere with experiments looking for extremely rare events.
Placing a detector deep underground greatly reduces this background. The surrounding rock absorbs most cosmic rays before they reach the equipment, helping researchers distinguish potentially interesting events from ordinary particle interactions.
Inside DAMA/LIBRA are highly purified sodium-iodide crystals containing a small amount of thallium. When certain particles deposit energy in these crystals, they can produce tiny flashes of light. Sensitive instruments measure that light, allowing researchers to study the energy and timing of individual events.
The experiment is designed to look for a particular pattern rather than simply count every flash.
If dark-matter particles are passing through the Solar System and occasionally striking atomic nuclei, the event rate could vary as Earth moves around the Sun. The detector must therefore measure extremely small changes over many years while controlling for environmental conditions and instrumental effects.
This is a difficult task. The potential signal is faint, and ordinary background events can imitate some of its features.
What is dark matter?
Dark matter is the name scientists give to an unseen component of the universe whose existence is inferred primarily from gravitational effects.
Galaxies rotate in ways that cannot be fully explained by the visible matter alone. Observations of galaxy clusters, gravitational lensing and the large-scale structure of the universe also support the presence of additional matter that does not emit or absorb light in the way ordinary matter does.
Measurements of the cosmic microwave background indicate that dark matter accounts for roughly 27% of the universe’s total mass-energy budget, while ordinary matter makes up about 5%. The remaining majority is attributed to dark energy, a separate phenomenon associated with the universe’s accelerated expansion.
Despite this evidence, the microscopic identity of dark matter remains unknown.
One proposed candidate is the weakly interacting massive particle, or WIMP. Such a particle would interact so rarely with ordinary matter that enormous detectors and long observation periods might be required to catch an interaction.
Other possibilities include axions, ultralight particles and more complex forms of dark matter.
The DAMA/LIBRA experiment is particularly relevant to searches for particles that could interact with atomic nuclei or otherwise deposit small amounts of energy in its crystals.
However, dark matter has not been directly confirmed through a universally accepted laboratory detection. Its gravitational effects provide strong evidence that something is missing from our understanding of the universe, but identifying the underlying particle or particles remains an open problem.
The mysterious annual pattern
DAMA/LIBRA’s central claim is an annual modulation: the rate of certain low-energy events rises and falls with a period of approximately one year.
Why would dark matter produce such a pattern?
The Solar System moves through the Milky Way, where dark matter is thought to form an extended halo. Earth also travels around the Sun. As a result, Earth’s velocity relative to the surrounding dark-matter distribution changes during the year.
Under a commonly used model, Earth should encounter a slightly different flux of dark-matter particles at different times. If those particles occasionally interact with a detector, the event rate might show a seasonal variation.
For the standard model used in this interpretation, the expected modulation reaches a maximum around early June, although alternative dark-matter distributions and interaction mechanisms can change the predicted pattern.
DAMA/LIBRA has reported an annual modulation consistent with the characteristic features its researchers expect from dark matter. The collaboration has continued to defend that interpretation in its published analyses.
But a recurring annual pattern is not automatically proof of dark matter.
Environmental changes, detector behaviour and other background processes must be examined carefully. Researchers also need independent experiments to test whether the signal can be reproduced under comparable conditions.
That is where the controversy begins.
Why other experiments have challenged the result
In particle physics, an extraordinary result becomes much more convincing when independent researchers can reproduce it using separate equipment and analyses.
DAMA/LIBRA’s result has proved difficult to reconcile with findings from other dark-matter searches. Many experiments using different detector materials have not found signals compatible with the simplest interpretations of DAMA/LIBRA’s measurements.
Differences between detector materials complicate the comparison because a hypothetical dark-matter particle might interact differently with different atomic nuclei. A more direct test therefore uses the same target material.
That is the strategy adopted by experiments including ANAIS-112 and COSINE-100, both of which use sodium-iodide crystals.
ANAIS-112 reports a significant disagreement
In a paper published in Physical Review Letters in 2025, the ANAIS-112 collaboration analysed six years of data from its sodium-iodide detectors at the Canfranc Underground Laboratory in Spain.
The researchers reported that their results were incompatible with the modulation signal reported by DAMA/LIBRA at a confidence level of four standard deviations under their analysis.
This is a substantial disagreement. It challenges the interpretation that the DAMA/LIBRA pattern is caused by the standard dark-matter interaction scenarios being tested.
It does not mean that all forms of dark matter have been ruled out. The result tests a particular signal and its interpretation, not every possible dark-matter model.
COSINE-100 reaches a similar conclusion
COSINE-100, another experiment using sodium-iodide detectors, has also reported results challenging the DAMA/LIBRA interpretation.
Its analysis of 6.4 years of data found no evidence of a comparable annual modulation signal. The collaboration reported that its results disfavoured the dark-matter explanation at greater than three standard deviations in the scenarios examined.
The importance of these findings is that the experiments use the same broad detector material as DAMA/LIBRA, reducing some of the uncertainties that arise when comparing completely different technologies.
However, differences in crystal properties, energy calibration, background modelling and detector response still require careful study.
The scientific disagreement has therefore not been reduced to a simple question of whether one team is correct. Researchers are investigating why the measured patterns differ and what those differences reveal about the experiments.
Could the signal be caused by something other than dark matter?
Yes. A measured pattern can be genuine without having the explanation initially proposed for it.
DAMA/LIBRA has argued that its annual modulation satisfies the expected characteristics of a dark-matter signal and that known background effects do not adequately explain it. Other collaborations have reported findings that challenge this interpretation.
Potential sources of uncertainty in rare-event experiments include radioactive contamination, detector response, changing environmental conditions, electronic effects and the way energy deposited in a detector is converted into a measured signal.
The details matter because the expected dark-matter signal is extremely small. A subtle change in detector behaviour can become important when the measurement relies on detecting a tiny annual difference in event rates.
This does not establish that a particular background effect is responsible for DAMA/LIBRA’s result. The cause of the disagreement remains a scientific question, and competing explanations must be tested against the data.
The responsible conclusion is that the annual modulation has been observed by DAMA/LIBRA, but its interpretation as a direct dark-matter detection has not been independently confirmed.
That distinction is essential. An unexplained signal is an invitation to investigate, not permission to declare a discovery.
Is something from space passing through Earth?
The possibility of dark matter passing through Earth is not inherently extraordinary. If dark matter exists as a widespread halo around the Milky Way, Earth would be moving through that distribution as part of the Solar System.
Many proposed dark-matter candidates would interact so weakly with ordinary matter that enormous numbers could pass through Earth without producing detectable effects.
But the DAMA/LIBRA controversy does not establish that an unknown force is interacting with the planet. Nor does it demonstrate that an exotic particle has been caught passing through the detector.
The experiment measures energy deposits in crystals. Researchers then compare the timing and characteristics of those events with predictions from physical models.
If a dark-matter explanation is eventually established, it would represent a major breakthrough in understanding the universe. If the signal turns out to have another origin, that would still be useful: it would help scientists improve rare-event experiments and refine their methods.
For now, the evidence does not justify claims that Earth is being affected by a mysterious new force or that the signal poses a threat to people.
Why the underground setting matters
Underground experiments are not built because scientists expect dark matter to be dangerous. They are built because the signal being sought is so difficult to distinguish from ordinary particle activity.
The rock above an underground laboratory acts as a natural shield against much of the cosmic-ray background. Additional layers of shielding and careful material selection reduce other sources of interference.
Researchers also monitor conditions that could affect their measurements. Long-running experiments need stable instruments, consistent calibration and detailed records of changes in the environment.
These safeguards make it possible to search for extremely rare events, but they cannot remove every uncertainty. That is why independent replication is so important.
A result that appears in one detector but not in another may reflect an unexpected property of the signal, a difference between instruments, a background effect or an incomplete theoretical model. Determining which explanation is correct requires more evidence.
What happens next in the search for dark matter?
The next stage involves combining long-term measurements with increasingly sensitive experiments and better ways to compare their results.
ANAIS-112 and COSINE-100 have already provided important tests of DAMA/LIBRA’s annual modulation. Other experiments are pursuing different dark-matter candidates and interaction mechanisms, using technologies designed to cover different ranges of possible particle masses and energies.
The COSINUS experiment is another effort to test the DAMA/LIBRA claim using sodium-iodide crystals at cryogenic temperatures. By measuring both scintillation light and phonon signals, it aims to improve particle identification and reduce some of the uncertainties associated with conventional sodium-iodide detectors.
Different approaches are valuable because dark matter may not behave exactly as the simplest models predict. No single experiment can explore every possible candidate and interaction.
The ultimate goal is to find a signal that can be independently reproduced and shown to have properties that distinguish it from known backgrounds.
That may require improved detectors, additional years of data, new theoretical models or a combination of all three.
What would a confirmed detection mean?
A convincing direct detection of dark matter would be a major scientific milestone.
It could help reveal whether dark matter is made of a new elementary particle, how that particle interacts with ordinary matter and whether it belongs to a broader family of particles beyond the Standard Model of particle physics.
The consequences would extend beyond laboratory physics. A better understanding of dark matter could improve models of galaxy formation and the evolution of cosmic structure.
But a detection claim would need more than an unexplained excess of events. Scientists would want independent evidence, careful scrutiny of possible backgrounds and a physical explanation consistent with other observations.
The history of science shows why that standard matters. Signals can point researchers toward new discoveries, but the process of verification is what separates a promising clue from a reliable conclusion.
DAMA/LIBRA remains an important example of this challenge.
The mystery is real—but the answer is still open
Deep underground in Italy and Spain, sensitive detectors are searching for evidence of matter that cannot be seen through ordinary light. One experiment has reported a persistent annual pattern that it interprets as a dark-matter signal. Other experiments using the same target material have failed to reproduce it.
That disagreement is scientifically significant. It is also unresolved in the sense that the community has not reached a universally accepted explanation for DAMA/LIBRA’s observations.
But it is not evidence of a mysterious force threatening Earth, and it does not establish that an unknown particle has been confirmed.
The most important lesson is about how science handles uncertainty. A result can be intriguing, repeatable within one experiment and still require independent confirmation. Conflicting measurements are not a reason to abandon the investigation; they are a reason to examine the methods and assumptions more closely.
Dark matter remains one of the biggest unanswered questions in modern physics. The underground signal is one piece of that larger puzzle—not yet the solution.
Frequently asked questions
Have scientists detected dark matter?
No direct dark-matter detection has been universally accepted by the scientific community. Evidence from gravity strongly supports the existence of dark matter, but its microscopic identity remains unknown.
What is the DAMA/LIBRA experiment?
DAMA/LIBRA is an underground experiment at Italy’s Gran Sasso National Laboratory that uses sodium-iodide crystals to search for rare particle interactions. It has reported an annual modulation in low-energy events that its researchers interpret as a possible dark-matter signal.
Why is the DAMA/LIBRA result controversial?
Other experiments using sodium-iodide crystals, including ANAIS-112 and COSINE-100, have not reproduced the same annual modulation. Their published results challenge the standard dark-matter interpretation of DAMA/LIBRA’s measurements.
Is the unexplained signal dangerous?
No known danger to Earth or humans has been identified. The experiments measure rare energy deposits in shielded detectors; they have not established that an unknown force is affecting the planet.
Why are dark-matter experiments built underground?
The rock above underground laboratories shields detectors from much of the cosmic-ray background. This helps researchers search for rare interactions that would otherwise be obscured by more common particle events.
When will scientists solve the mystery?
There is no reliable timeline. Progress depends on new data, improved detectors, careful comparison between experiments and the development of explanations that can be tested independently.
Editorial disclaimer: This article describes ongoing research into dark matter and the disputed annual modulation reported by DAMA/LIBRA. The signal has not been independently confirmed as a dark-matter detection. Scientific interpretations may change as new measurements become available.
References and further reading
- Amaré et al., Towards a Robust Model-Independent Test of the DAMA/LIBRA Dark Matter Signal: ANAIS-112 Results with Six Years of Data, Physical Review Letters (2025). Read the paper.
- COSINE-100 Collaboration, COSINE-100 Full Dataset Challenges the Annual Modulation Signal of DAMA/LIBRA. Read the study.
- DAMA Collaboration, DAMA/LIBRA experiment overview.
- Gran Sasso National Laboratory, Laboratori Nazionali del Gran Sasso.
- CERN, Dark matter research.
