Dark Matter & Dark Energy

95% of the universe is invisible. Dark matter holds galaxies together and builds cosmic structure. Dark energy is tearing the universe apart. Both are among the most firmly established facts in physics — and both are completely unexplained.

1. Evidence for Dark Matter

Dark matter doesn't emit, absorb, or reflect light — but it gravitates. Multiple independent lines of evidence, all pointing to the same conclusion:

Galaxy rotation curves (Rubin & Ford, 1970s): Stars in the outer disks of galaxies orbit at roughly constant velocity, independent of distance from the centre — far faster than visible matter alone would produce. A dark matter halo extending well beyond the visible disk matches observations.
Gravitational lensing: Galaxies and clusters bend light more than their visible mass allows. Mapping the lensing signal reveals a dark matter distribution extending far beyond the stellar component.
Galaxy cluster dynamics (Zwicky, 1933): Galaxies in clusters move far too fast to be bound by the visible mass. The "missing mass" was identified — now called dark matter.
Cosmic structure: The observed galaxy distribution, BAO, and cluster mass function match N-body simulations with cold dark matter but not without it.
CMB power spectrum: The relative heights of acoustic peaks constrain the baryon density and total matter density independently — requiring ~5× more dark matter than baryons.

Expected vs observed rotation velocity Keplerian (visible only): v \propto 1/\sqrtr for large r Observed (with dark matter): v \approx constant (flat rotation curve) Dark matter halo density: ρ(r) \propto 1/r² (isothermal sphere) \to total mass M(r) \propto r \to v = \sqrt(GM/r) = constant ✓

2. The Bullet Cluster

The Bullet Cluster (1E 0657-558) is the clearest single piece of dark matter evidence. Two galaxy clusters collided ~150 million years ago. The collision separated three components:

The stars in each cluster passed through relatively unimpeded (stars rarely collide even in galaxy mergers).
The hot gas (visible in X-ray) was slowed by electromagnetic drag during the collision. The gas (90% of the baryonic mass) now lags behind the stars.
The dark matter (mapped by gravitational lensing) is co-located with the stars — not the gas — indicating it is weakly self-interacting and collisionless.

This directly shows that most of the cluster mass is not the X-ray emitting gas, but a collisionless component that passed through — dark matter. Modified gravity theories struggle to explain this.

3. Dark Matter Candidates

WIMPs

~10–1000 GeV/c²

Weakly Interacting Massive Particles. Predicted by supersymmetry. Natural "WIMP miracle" abundance matches cosmological Ω_DM. No detection despite extensive direct search (LUX-ZEPLIN, XENONnT, PandaX).

Axions

~10⁻⁵–1 eV/c²

Ultra-light particles proposed to solve the strong CP problem. Could form a Bose-Einstein condensate. ADMX experiment searches for axion-photon conversion in a magnetic field. Active search.

Sterile Neutrinos

~keV range

Right-handed neutrinos not coupling to W/Z bosons. Could explain X-ray line at 3.5 keV seen in clusters. Disputed.

Primordial Black Holes

variable

Could form from early-universe density fluctuations. LIGO and microlensing surveys constrain mass range. Still viable in ~10⁻¹⁵–10⁻¹¹ solar mass window.

4. Dark Energy

Dark energy is not dark matter's companion concept — they are completely different phenomena. Dark energy is a negative-pressure component driving the accelerated expansion of the universe. Its simplest form is Einstein's cosmological constant Λ — a constant energy density of empty space.

Equation of state w = p / (ρc²) Cosmological constant (Λ): w = -1 (exactly) Quintessence (dynamic field): w \neq -1, may evolve Current best-fit: w = -1.03 \pm 0.03 (consistent with Λ)

The energy density of a cosmological constant scales as a⁰ — constant regardless of expansion. In contrast, matter dilutes as a⁻³ and radiation as a⁻⁴. At the current expansion factor, dark energy density equals matter density (~z ≈ 0.4), after which expansion accelerates.

The cosmological constant problem: Quantum field theory predicts vacuum energy density ~10¹²⁰ times larger than observed. This is arguably the worst prediction in physics. Either there is a mechanism that cancels the QFT contribution, or the cosmological constant is anthropically selected, or dark energy is not Λ.

5. Cosmic Acceleration

In 1998, two independent supernova survey teams (Riess et al.; Perlmutter et al.) used Type Ia supernovae as standard candles. They found distant supernovae were ~25% fainter than expected — the universe's expansion is accelerating. This won the 2011 Nobel Prize in Physics.

Independent confirmations: CMB acoustic peaks, BAO feature positions, galaxy cluster counts vs redshift. All agree: the universe has been accelerating for the past ~5 billion years.

6. Alternative Explanations?

MOND (Modified Newtonian Dynamics, Milgrom 1983): Instead of dark matter, modify gravity: below a critical acceleration a₀ ≈ 1.2×10⁻¹⁰ m/s², gravity falls off as 1/r rather than 1/r². MOND fits individual galaxy rotation curves with one parameter. But: fails for galaxy clusters (still needs dark matter there), and the Bullet Cluster is difficult to explain. The relativistic extension (TeVeS) has further problems and is increasingly constrained by gravitational wave observations.

For dark energy: Modified gravity (f(R) theories, scalar-tensor theories), extra dimensions (DGP brane model), and quintessence (evolving scalar field) all attempt to explain acceleration without Λ. Current data is consistent with Λ; new surveys (Euclid, DESI, Rubin) aim to constrain w with 1% precision.

7. Current Searches

Direct detection: LUX-ZEPLIN (10 tonnes of liquid xenon), XENONnT at Gran Sasso — look for recoiling nuclei from WIMP-nucleus scattering.
Indirect detection: Search for WIMP annihilation products (gamma rays, positrons, neutrinos) with Fermi-LAT, AMS-02, IceCube.
Collider production: LHC searches for missing transverse energy signatures of dark matter pair production.
Axion searches: ADMX (USA), HAYSTAC, ABRACADABRA, CASPEr — magnetic resonance and photon conversion techniques.
Dark energy surveys: DESI (spectroscopy of 40 million galaxies), Euclid (weak lensing + galaxy clustering), Rubin/LSST (photometric survey).

Status 2026: No confirmed direct detection of any dark matter particle after ~30 years of searching. Parameter space for classic WIMPs is increasingly constrained. The field is shifting toward lighter candidates (light DM, axions, dark photons) and more exotic scenarios.