Dark Matter — What We Actually Know (and Don't)

# Dark Matter — What We Actually Know (and Don't)

About 27% of the universe is dark matter. We know this with reasonable confidence. We have essentially no idea what it is. This isn't a crisis in physics — it's the most interesting open problem in the field.

## The Evidence That Made Dark Matter Mainstream

### Galaxy Rotation Curves
In the 1970s, Vera Rubin (and Fritz Zwicky decades earlier) noticed something strange: stars at the outer edges of galaxies orbit at roughly the same speed as stars near the center. In Newtonian gravity with visible matter only, outer stars should orbit more slowly — like the outer planets in our solar system orbit the Sun more slowly than the inner ones.

The only explanation that works is that galaxies are embedded in a large sphere of additional, invisible mass — the "dark matter halo." This halo extends far beyond the visible stellar disk, providing gravitational scaffolding that explains the flat rotation curves.

### Gravitational Lensing
General relativity predicts that massive objects bend light. When we observe clusters of galaxies, the lensing effect is far stronger than the visible matter can account for. The "Bullet Cluster" is the canonical example: two galaxy clusters that collided. The hot gas (visible via X-ray) was slowed by the collision, while the dark matter (inferred from lensing) passed straight through — exactly what a non-interacting dark matter halo would do.

### Large-Scale Structure
The distribution of galaxies in cosmic web filaments and voids matches simulations only when dark matter is included. Without it, galaxies wouldn't have the gravitational seeds to cluster as we observe them.

## What Dark Matter Probably Isn't

The strongest candidates have been ruled out one by one:

- **MACHOs (Massive Compact Halo Objects)**: Black holes, neutron stars, brown dwarfs in halos. Microlensing surveys have largely ruled these out at the scales that would explain the observations.
- **Neutrinos**: Too light and too fast (hot dark matter) — they'd smear out small-scale structure rather than help form it.
- **Primordial black holes (most mass ranges)**: Gravitational wave surveys and microlensing have constrained most mass windows.

## The Leading Candidates

### WIMPs (Weakly Interacting Massive Particles)
For 30 years the dominant candidate. A particle of ~100 GeV that interacts only via the weak force and gravity. LHC searches and direct detection experiments (LUX, XENON1T, PandaX) have found nothing. The WIMP parameter space is significantly constrained but not fully excluded.

### Axions
Originally proposed to solve the strong CP problem in QCD. Light (~10⁻⁵ eV), extremely weakly interacting. Axion dark matter would form a coherent wave-like field rather than individual particles. ADMX and HAYSTAC experiments are probing viable axion mass ranges. This is arguably the most actively growing experimental area.

### Sterile Neutrinos
A hypothetical right-handed neutrino that doesn't interact via the Standard Model forces. Could explain the X-ray line at 3.5 keV seen in galaxy cluster observations — though that signal remains contested.

## The Alternative: Modified Gravity

MOND (Modified Newtonian Dynamics) and its relativistic extensions (TeVeS, RMOND) attempt to explain galaxy rotation curves without dark matter by modifying gravity at low accelerations. These theories work well for individual galaxies but fail for galaxy clusters without reintroducing dark matter anyway. Most physicists consider modified gravity a partial description, not a replacement.

## Where We Are

No dark matter particle has been directly detected. The null results from direct detection experiments are informative — they rule out large swaths of parameter space — but they don't tell us what dark matter *is*. The axion path looks increasingly promising and the next generation of experiments (ADMX-Sidecar, ABRACADABRA, CASPEr) will probe significant new territory in the next decade.

The honest answer in 2026: dark matter exists, it dominates galactic structure, and we don't know what it's made of. That's simultaneously the most embarrassing and most exciting situation in physics.

Dark Matter — What We Actually Know (and Don't)

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