Enigma of the Missing Mass: Dark Matter and Dark Energy

Dark matter and dark energy in the cosmos — WMAP sky analyses indicate that the Universe is 13.8 billion years old (with 1% precision), composed of 68.3% dark energy, 26.8% cold dark matter, and only 4.9% atoms. Map of cosmic microwave background radiation dating back to about 380,000 years after the Big Bang. The colors are artificial and show tiny temperature variations. Image source: (public domain) WMAP Science Team, NASA

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)
Component	Proportion	Physical Nature	Detection Method
Baryonic Matter	4.9%	Atoms, plasma, dust	Emission, absorption, visible light, X-rays
Dark Matter	26.8%	Unknown (WIMPs? axions? neutrinos?)	Gravitational effects (slowness of galactic rotation speeds, gravitational lenses)
Dark Energy	68.3%	Unknown (quantum vacuum? cosmological constant?)	Acceleration of cosmic expansion, Type Ia supernovae

Source: ESA Planck Mission (new window), 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
Candidate	Nature	Theoretical Origin	Experimental Status
WIMPs	Weakly Interacting Massive Particles	Supersymmetry (SUSY), neutralinos	Not detected (XENONnT, LUX-ZEPLIN)
Axions	Very light, pseudoscalar particles	Peccei-Quinn theory, QCD	Ongoing searches (ADMX, MADMAX)
Sterile Neutrinos	Inert neutrinos interacting only through gravity	Extension of the Standard Model	Not confirmed, but compatible with certain anomalies
Self-Interacting Dark Matter (SIDM)	Particles that annihilate or scatter among themselves	Galactic structure models	Tests on cluster density profiles
Ultralight Dark Matter (Fuzzy DM)	Bosons with mass ~\(10^{-22}\) eV	Cosmological Bose-Einstein condensate	Effects on large-scale halos
MACHOs	Non-luminous compact astrophysical objects	Black holes, brown dwarfs, dead stars	Ruled out for most of the dark matter (microlensing)
MOND / TeVeS	Modifications of Newtonian dynamics	Alternative theories to dark matter	Inability to reproduce gravitational lenses
Hidden Sector Dark Matter Particles	Particles interacting via non-standard forces (e.g., dark photons)	Extensions of the Standard Model, sometimes from string theory	Very weak interactions with our sector, ongoing indirect searches
Primordial Black Holes (PBH)	Black holes formed during the inflationary era	Inflation models with local density fluctuations	Gravitational microlensing, gravitational waves (LIGO/Virgo)
Mirror Matter	Replica of the Standard Model in a mirror sector	Inspired by \(\mathbb{Z}_2\) symmetry, respect for parity	Potentially detectable via neutrino oscillations or thermal effects

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

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
Model	Description	Component (\(w\))	Experimental Tests
Cosmological Constant (\(\Lambda\))	Vacuum energy, constant density	\(w = -1\)	Agreement with Planck, SDSS, SN Ia data
Quintessence	Dynamic scalar field with slow potential	\(-1 <w <-\frac{1}{3}\)	Euclid, DESI, LSST projects
Phantom Energy	Negative energy field	\(w <-1\)	Would lead to the Big Rip scenario
k-essence	Scalar field with non-standard kinetics	Variable, depends on dynamics	Predictions for Euclid and LSST
Modified Gravity (f(R), DGP…)	Modification of general relativity on large scales	No defined \(w\) (geometric effect)	Tests on the formation of large structures
Cosmological Backreaction	Emergent effect of gravitational inhomogeneities	No fluid, but average geometry	Very difficult to model, no consensus
Emergent Dark Energy (Entropic Gravity)	Gravity and acceleration as thermodynamic phenomena	No 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 radius	Tests on CMB, structure formation, upcoming LSST
Dark Vector Fields	Dynamic vectors responsible for the acceleration of expansion	\( w(t) \) variable according to the field	Testable models by anisotropies of the diffuse background
Delayed Interaction Theories	The effect of matter on the geometry of spacetime is not instantaneous	No fluid, memory or delayed effect of ordinary matter	Speculative hypothesis, difficult to test experimentally

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