Last updated September 5, 2025

What is Dark Matter? The Invisible Force Shaping the Universe

Cosmic Mystery

Dark matter is one of the greatest mysteries of modern physics. Invisible, it emits neither light nor detectable radiation, but its gravitational influence is measurable throughout the Universe. According to current cosmological models, about 27% of the Universe's energy content consists of this unknown matter, compared to just 5% for ordinary baryonic matter.

History of Discovery

The existence of dark matter was first suggested by Fritz Zwicky (1898-1974) in 1933, while studying the velocities of galaxies in the Coma Cluster. He found that the visible mass was insufficient to keep the cluster gravitationally bound. Later, in the 1970s, Vera Rubin (1928-2016) measured the rotation curve of spiral galaxies. Contrary to Kepler's law, which predicts a decrease in velocity with radius, Rubin observed that the velocity stabilized, implying the presence of a dominant invisible mass.

The Spectacular Evidence of the Bullet Cluster (see image)

Context and Discovery

The Bullet Cluster (1E 0657–56), located about 3.7 billion light-years away, is considered direct evidence of dark matter's existence. This cluster is the result of the collision of two galaxy clusters (see image), whose analysis combines X-ray data (in red) and gravitational lensing effects (in blue).

X-ray Observations

Observations by the Chandra satellite show extremely hot gas (temperature > 10⁸ K), which radiates intensely in X-rays. This gas (in red), which makes up most of the visible baryonic mass of the clusters, is strongly slowed by pressure forces during the collision. It thus accumulates in the center, between the two clusters.

Gravitational Lensing Mapping

The total mass distribution was measured using the gravitational lensing effect on background galaxies. Surprisingly, the gravitational mass is offset from the baryonic gas and is concentrated around the galaxies themselves, on either side of the gas region (in blue).

Physical Interpretation

This clear separation between baryonic matter (hot gas, red in the image) and the dominant gravitational mass (dark matter, blue) is considered irrefutable proof that the observed gravity cannot be explained by visible matter alone. It also contradicts modified gravity models like MOND, which predict that the mass distribution should follow that of baryonic matter.

The Bullet Cluster thus stands as a cornerstone in favor of the standard cosmological model, known as ΛCDM, where about 27% of the Universe's energy content is attributed to dark matter.

Cosmological Consequences

Dark matter plays a central role in the evolution of the Universe:

It structures galaxies by stabilizing their halos.
It accelerates the formation of large-scale structures.
It is integral to the ΛCDM (Lambda Cold Dark Matter) model, the standard model of cosmology.
It complements dark energy, which makes up 68% of the cosmos.

Comparative Table of Theoretical Hypotheses on Dark Matter

Detailed comparison of dark matter hypotheses
Hypothesis	Physical Nature	Typical Mass	Domain of Action	Advantages	Limitations
WIMPs	Weakly interacting massive particles	10 GeV – 10 TeV	Cosmology and galactic dynamics	Predicted by extensions of the Standard Model (SUSY), abundance compatible with thermal calculations from the Big Bang	Not detected despite decades of direct and indirect experiments
Axions	Ultra-light particles	10^-6 to 10^-2 eV	Cosmology, stellar astrophysics	Can solve the strong CP problem: the introduction of a Peccei-Quinn symmetry breaks spontaneously, driving the QCD θ parameter to zero; the quantum excitation of this field is the axion	Depend on photon-axion conversion experiments (ADMX, CAST), uncertain cosmological abundance parameter
MACHOs	Massive compact halo objects (brown dwarfs, primordial black holes)	0.1 – 10 M☉	Galactic halos	Simple astrophysical explanation, observable via gravitational microlensing	Population too small to explain total dark matter, statistically limited detection
Sterile Neutrinos	Hypothetical non-interacting neutrinos	keV – MeV	Cosmology, structure formation	Can explain warm dark matter, affect the formation of small structures	Constrained by X-ray radiation and cosmological data, still speculative model
MOND	Modification of Newton's laws at low acceleration	No particle	Galactic dynamics	Accurately explains galactic rotation curves without dark matter	Incompatible with cosmology (CMB, cluster formation), no physical detection
Additional Exotic Particles	E.g., gravitinos, superparticles from SUSY	10 GeV – 1 TeV	Early universe, galactic halos	Can be long-term stable and explain observed abundance	Extremely difficult to detect, highly model-dependent

Sources: Bertone & Tait, Nature 2022, Particle Data Group 2024, ESA Planck Mission, Bahcall et al. 1999.