The Quest for Dark Matter

9 October 2025

Dark matter is a form of matter that is invisible to telescopes, as it does not emit electromagnetic radiation, hence we call it “dark.” Its (presumed) existence can currently be detected only indirectly, through its gravitational effects.

Despite the many experiments around the world currently trying to detect its traces, including those currently collecting data at the INFN Gran Sasso National Laboratory, dark matter has not yet been experimentally observed. Detecting and studying dark matter is one of the fundamental challenges of modern physics.

Until the first half of the 20th century, it was believed that almost all of the mass in the Universe was concentrated in stars. Today, however, we know that stars account for only a tiny fraction of cosmic matter. In fact, the energy balance of the universe, derived from measurements of the cosmic microwave background, indicates that ordinary matter makes up less than 5% of the total. The remaining 95% is divided between dark matter (about 27%) and dark energy (about 68%).

While the nature of dark energy, a sort of negative pressure responsible for the accelerated expansion of the universe, is highly speculative, dark matter could consist of either macroscopic components, such as black holes, or microscopic ones, such as elementary particles. To account for the formation of large-scale structures in the universe, these particles must be electrically neutral, massive, and weakly interacting.

Nevertheless, there are no particles with these characteristics in the Standard Model of particle physics, just as there is no explanation for the observed asymmetry between matter and antimatter in the cosmos.

The energetic budget of the universe

Underground entrance of the INFN Gran Sasso National Laboratory

In recent decades, theoretical physicists have proposed a large number of models attempting to explain dark matter. One of the most popular suggests the existence of so-called WIMPs (Weakly Interacting Massive Particles), hypothetical particles that are relatively heavy and interact only weakly, that interact with ordinary matter only through the weak force, and which, for various reasons, would have almost perfect characteristics to account for dark matter.

Particles with these features would have been easily produced in the hot plasma that made up the early universe. As the universe cooled, they would have decoupled from the plasma and, if stable, could have survived to the present day. The DarkSide-20k and XENONnT experiments, currently collecting data in the cosmic silence of the INFN Gran Sasso National Laboratory, are searching for WIMPs using different and competing technologies.

The discovery of a new particle with WIMP-like properties would represent an extraordinary breakthrough—not only for particle physics (by revealing physics beyond the Standard Model), but also for cosmology.

Among the many particles hypothesized to account for the composition of mysterious dark matter, the axion was originally introduced to solve a fundamental problem in the theory of strong interactions. The axion is thought to have a mass much smaller than that of the electron, to be electrically neutral, to have a lifetime longer than the age of the universe (13.8 billion years), to interact extremely weakly with ordinary matter, and to possess no spin.

Axions are believed to have been abundantly produced in the very early moments after the Big Bang, and to have spread throughout the universe like a fluid—similar to what happened with the primordial photon gas that today makes up the cosmic microwave background. However, unlike those photons, axions have mass, and could therefore account for the universe’s missing mass, dark matter, of which we have much indirect evidence.

This fascinating hypothesis is discussed in the October issue of Particle Chronicle, the INFN newsletter, in an interview with Pierre Sikivie, Professor of Physics at Florida University and recipient of the 2025 Galileo Galilei Medal.

First hypothesized in 1933 by Austrian astronomer Fritz Zwicky, dark matter only truly entered the scientific debate in the 1970s, thanks to observations of the Andromeda galaxy made by American astronomers Vera Rubin and Kent Ford.

The two scientists measured the rotational speed of stars at the outskirts of the galaxy and found values surprisingly high compared to predictions based on Newtonian gravity. The most plausible explanation for this anomaly was the existence of an invisible form of matter, hence “dark” matter, whose contribution (in addition to that of ordinary matter) would account for the observed velocity.

Since the discovery by Rubin and Ford, the dark matter hypothesis has become increasingly popular within the scientific community, also thanks to other indirect evidence beyond the problem of galaxy rotation speeds, such as gravitational lensing, the observation of the Bullet Cluster, and data related to the cosmic microwave background radiation.

A composite image of the Bullet Cluster.
The Bullet Cluster is a pair of galaxy clusters, which have collided head on. The optical image from the Magellan and the Hubble Space Telescope shows galaxies in orange and white in the background. Hot gas, which contains the bulk of the normal matter in the cluster, is shown by the Chandra X-ray image, which showst the hot intracluster gas (pink). Gravitational lensing, the distortion of background images by mass in the cluster, reveals the mass of the cluster is dominated by dark matter (blue), an exotic form of matter abundant in the Universe, with very different properties compared to normal matter.
This was the first clear separation seen between normal and dark matter. (© NASA/CXC/M. Weiss)

According to current estimates, dark matter makes up about 85% of the total mass of the universe, while ordinary matter, the kind we are made of and are familiar with, accounts for only the remaining 15%. In other words, it’s not just a dark mystery, but a giant one.

To date, there is no confirmed observational evidence for WIMPs or any other hypothetical dark matter particles. While the scientific community remains committed to pushing forward with even more sensitive experimental searches, alternative hypotheses to dark matter are also beginning to be taken more seriously such as those related to primordial black holes or the theory of modified gravity.

Since the discovery by Rubin and Ford, the dark matter hypothesis has become increasingly popular within the scientific community, also thanks to other indirect evidence that goes beyond the issue of galaxy rotation curves, such as gravitational lensing, the observation of the Bullet Cluster, data from the cosmic microwave background radiation, and insights from simulations.

Gravitational lensing is a phenomenon predicted by Einstein’s theory of general relativity, and it is observed when the light from a distant object (such as a galaxy or a supernova) is bent by the gravity of a very massive object (like another galaxy or a galaxy cluster) that lies between the observer and the distant object. Although dark matter does not emit light or electromagnetic radiation, it plays a fundamental role in gravitational lensing. In fact, dark matter contributes to the mass of the lensing object, thereby increasing the curvature of space-time and altering the path of the light coming from background objects.

The Bullet Cluster is a system composed of two galaxy clusters that collided at high speed about 3 billion light-years away from us. During the collision, the different components of the cluster behaved in different ways, supporting the hypothesis of a large amount of invisible mass attributable to dark matter. The Bullet Cluster shows that most of the universe’s mass is not made of visible matter and is one of the most direct observational proofs of the existence of dark matter, as it reveals a physical separation between visible matter and gravitational mass.

Moreover, the study of the Cosmic Microwave Background (CMB), a microwave radiation that permeates the entire universe and was released about 380,000 years after the Big Bang, when the universe was still in its infancy, provides important evidence for the existence of dark matter. In particular, the CMB offers precise information about the density and composition of the early universe, including the amounts of ordinary and dark matter present shortly after the Big Bang, a sort of fingerprint of the primordial universe.

Among the supporting evidence, one can also include the existence of large-scale structures in the universe, whose formation, according to numerical simulations, appears to require a greater amount of mass than what is directly observable.