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Last updated August 1, 2025

Enigma of the Missing Mass: Dark Matter and Dark Energy

Dark matter and dark energy in the cosmos

The Mystery of the 95% Invisible Matter and Energy

When we observe galaxies, galaxy clusters, or the cosmic microwave background, one thing is clear to cosmologists: the laws of gravity are not sufficient to explain the observations. However, visible matter, which makes up stars, planets, and ourselves, accounts for only 4.9% of the universe. The remaining 95.1% consists of two invisible and mysterious entities: dark matter and dark energy.

Summary Table of the Universe's Components

Estimated Composition of the Universe (Planck 2018)
ComponentProportionPhysical NatureDetection Method
Baryonic Matter4.9%Atoms, plasma, dustEmission, absorption, visible light, X-rays
Dark Matter26.8%Unknown (WIMPs? axions? neutrinos?)Gravitational effects (slowness of galactic rotation speeds, gravitational lenses)
Dark Energy68.3%Unknown (quantum vacuum? cosmological constant?)Acceleration of cosmic expansion, Type Ia supernovae

Source: ESA Planck Mission, 2018 Results.

Dark Matter: An Invisible Gravitational Halo

Inferred from galactic rotation and cluster dynamics, dark matter accounts for about 26.8% of the total energy of the universe. It does not emit, absorb, or reflect light. Its presence is only betrayed by its gravitational effects, such as the abnormal slowness of the decrease in the speed of stars with distance from the galactic center. Various hypotheses have been proposed: massive neutrinos, axions, WIMPs (Weakly Interacting Massive Particles), but no candidate has been detected to date.

Theories on the Nature of Dark Matter

The lack of direct detection of dark matter stimulates a variety of theoretical models, ranging from supersymmetric particles to extensions of the gravitational field. These hypotheses seek to explain its properties: gravitational but not electromagnetic, massive but not visible.

Theoretical Hypotheses on Dark Matter
CandidateNatureTheoretical OriginExperimental Status
WIMPsWeakly Interacting Massive ParticlesSupersymmetry (SUSY), neutralinosNot detected (XENONnT, LUX-ZEPLIN)
AxionsVery light, pseudoscalar particlesPeccei-Quinn theory, QCDOngoing searches (ADMX, MADMAX)
Sterile NeutrinosInert neutrinos interacting only through gravityExtension of the Standard ModelNot confirmed, but compatible with certain anomalies
Self-Interacting Dark Matter (SIDM)Particles that annihilate or scatter among themselvesGalactic structure modelsTests on cluster density profiles
Ultralight Dark Matter (Fuzzy DM)Bosons with mass ~\(10^{-22}\) eVCosmological Bose-Einstein condensateEffects on large-scale halos
MACHOsNon-luminous compact astrophysical objectsBlack holes, brown dwarfs, dead starsRuled out for most of the dark matter (microlensing)
MOND / TeVeSModifications of Newtonian dynamicsAlternative theories to dark matterInability to reproduce gravitational lenses
Hidden Sector Dark Matter ParticlesParticles interacting via non-standard forces (e.g., dark photons)Extensions of the Standard Model, sometimes from string theoryVery weak interactions with our sector, ongoing indirect searches
Primordial Black Holes (PBH)Black holes formed during the inflationary eraInflation models with local density fluctuationsGravitational microlensing, gravitational waves (LIGO/Virgo)
Mirror MatterReplica of the Standard Model in a mirror sectorInspired by \(\mathbb{Z}_2\) symmetry, respect for parityPotentially detectable via neutrino oscillations or thermal effects

Sources: Bertone & Tait (2018), LUX-ZEPLIN Collaboration (2022), Fuzzy Dark Matter Review (2020).

Dark Energy: The Force of Cosmic Acceleration

Indirectly discovered by the study of Type Ia supernovae in the 1990s, dark energy would account for 68.3% of the universe. It would be responsible for the acceleration of cosmic expansion, like a form of cosmic antigravity. Modeled by the cosmological constant \(\Lambda\) in Einstein's equations, it defies our understanding of quantum fields and the vacuum itself. The density of this energy seems to remain constant over time, which makes its origin deeply enigmatic.

Theories on the Nature of Dark Energy

Dark energy is one of the major mysteries of cosmology. Several models seek to explain its repulsive effect on the expansion of space, either as an intrinsic property of the vacuum or as a manifestation of a deeper or geometric dynamic of the universe.

Theoretical Hypotheses on Dark Energy
ModelDescriptionComponent (\(w\))Experimental Tests
Cosmological Constant (\(\Lambda\))Vacuum energy, constant density\(w = -1\)Agreement with Planck, SDSS, SN Ia data
QuintessenceDynamic scalar field with slow potential\(-1 < w < -\frac{1}{3}\)Euclid, DESI, LSST projects
Phantom EnergyNegative energy field\(w < -1\)Would lead to the Big Rip scenario
k-essenceScalar field with non-standard kineticsVariable, depends on dynamicsPredictions for Euclid and LSST
Modified Gravity (f(R), DGP...)Modification of general relativity on large scalesNo defined \(w\) (geometric effect)Tests on the formation of large structures
Cosmological BackreactionEmergent effect of gravitational inhomogeneitiesNo fluid, but average geometryVery difficult to model, no consensus
Emergent Dark Energy (Entropic Gravity)Gravity and acceleration as thermodynamic phenomenaNo classical \(w\)Speculative theory, without precise predictions
Holographic Models (HDE)Dark energy arises from a holographic principle on information density\( w \approx -1 \), depends on the cosmological radiusTests on CMB, structure formation, upcoming LSST
Dark Vector FieldsDynamic vectors responsible for the acceleration of expansion\( w(t) \) variable according to the fieldTestable models by anisotropies of the diffuse background
Delayed Interaction TheoriesThe effect of matter on the geometry of spacetime is not instantaneousNo fluid, memory or delayed effect of ordinary matterSpeculative hypothesis, difficult to test experimentally

Sources: Copeland et al. (2006), ESA Euclid, Verlinde (2011) – Gravity as an Emergent Phenomenon.

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