60 Fun Facts About Dark Matter

Dark matter is so called because it does not interact with what?

Its existence is inferred entirely from gravitational effects.

In the standard Lambda-CDM model, roughly what share of the universe's mass-energy is dark matter?

Ordinary matter is about 5 percent and dark energy about 68 percent.

Dark matter makes up roughly what fraction of the total matter (not energy) in the universe?

Ordinary atoms account for the remaining 15 percent or so.

According to the article, all the dark matter out to Neptune's orbit would weigh about as much as what?

Its density in the Solar System is far lower than that of ordinary matter.

Which physicist speculated in 1884 that many stars near the Sun might be dark bodies?

He was estimating the density of stars from their velocity dispersion.

Which French mathematician used the term matière obscure in 1906?

He wrongly concluded there must be less of it than visible matter.

Fritz Zwicky applied the virial theorem to which galaxy cluster in 1933?

He estimated the cluster had about 400 times more mass than could be seen.

Why were Zwicky's 1933 mass estimates off by more than an order of magnitude?

Even so, he correctly concluded that most of the gravitating matter was dark.

In which city was Fritz Zwicky born?

His Swiss father was an industrialist there and served as Norwegian consul.

Which astronomical term did Zwicky coin while developing the idea of neutron stars?

He proposed them as giant explosions at the end of a star's life.

What was Zwicky's famous insult for people he disliked?

He explained they were bastards no matter which way you looked at them.

Which instrument-maker collaborated with Vera Rubin on her 1970s rotation-curve measurements?

They used his image-tube spectrograph on edge-on spiral galaxies.

What did Rubin observe about the outermost parts of spiral galaxies?

Kepler's laws predict speeds should fall with distance, as in the Solar System.

Rubin became the first woman permitted to observe at which observatory in 1965?

She had joined the Carnegie Institution of Washington that same year.

Under which famous physicist did Rubin earn her PhD at Georgetown in 1954?

She had turned down Harvard to follow her husband to Cornell.

Which honour is Rubin widely argued to have been unjustly denied?

She did receive the National Medal of Science and the RAS Gold Medal.

The Vera C. Rubin Observatory, named in her honour, is located in which country?

It sits on Cerro Pachón, beside the Gemini South telescope.

What record does the Rubin Observatory's 3.2-gigapixel imaging instrument hold?

The 3.2-gigapixel CCD camera images the whole southern sky every few days.

Two 1974 papers argued galaxies sit in halos of unseen matter. Which cities were the teams in?

The Estonian team was led by Jaan Einasto; Jim Peebles was on the American one.

Radio astronomers traced galaxy rotation far beyond the visible stars by mapping which emission?

Rotation curves were traced to 30 kiloparsecs by 1975, far past the optical disk.

Which colliding pair of galaxy clusters shows its lensing mass sitting apart from its visible gas?

Hot gas was slowed by the collision while the dark matter sailed straight through.

Which type of theory does the colliding-cluster evidence make particularly difficult to defend?

Modified gravity generally predicts lensing around the visible matter, not beside it.

Galaxy cluster masses measured three ways agree dark matter outweighs visible matter by roughly what ratio?

The methods are galaxy velocities, X-ray gas temperature and gravitational lensing.

What does 'cold' mean when cosmologists classify the unseen mass as cold rather than hot?

The categories refer to free streaming length, not actual temperature.

Why can neutrinos not make up the bulk of the universe's missing mass?

Hot dark matter would have formed superclusters first, but galaxies formed first.

What does the acronym WIMP stand for?

WIMPs are expected to interact via gravity and a force no stronger than the weak nuclear force.

Which substance fills the detectors of leading WIMP-hunting experiments such as LUX, PandaX and LZ?

All have so far reported null results across the GeV to TeV mass range.

What is the 'fog' or 'floor' that the LZ experiment entered in 2025?

LZ detected boron-8 solar neutrinos scattering off nuclei in the detector.

Which theory's failure to appear at the Large Hadron Collider has squeezed the case for WIMPs?

The null results have shifted attention toward axions and macroscopic candidates.

Frank Wilczek named the axion after what?

Steven Weinberg had wanted to call it the higglet.

The axion was proposed to solve which puzzle in quantum chromodynamics?

Roberto Peccei and Helen Quinn devised the mechanism in 1977.

How do experiments such as ADMX try to detect axions?

This conversion is known as the Primakoff effect.

What does MACHO stand for?

Candidates include black holes, neutron stars, brown dwarfs and rogue planets.

Besides new particles, what is the other main candidate for non-baryonic dark matter?

Interest revived after observations by LIGO and the James Webb Space Telescope.

Which Israeli physicist proposed MOND, modified Newtonian dynamics, in 1983?

He suggested gravity decays more slowly than Newton's law at very low accelerations.

What is the relativistic generalisation of MOND called?

None of these rivals yet explains every observation at once.

Dark energy, unlike dark matter, is chiefly credited with doing what?

Dark energy is thought to make up about 68 percent of the universe's mass-energy.

What role is dark matter thought to play in the large-scale structure of the cosmos?

It clumped along filaments into a cosmic web after the Big Bang.

Which effect, predicted by Nick Kaiser in 1987, makes superclusters look squashed in 3D galaxy maps?

It was first decisively measured by the 2dF Galaxy Redshift Survey in 2001.

Strong gravitational lensing distorts background galaxies into what shape around clusters like Abell 1689?

Weak lensing produces only minute distortions measured statistically.

A substance called Dust, inspired by the physics of unseen matter, features in which fantasy trilogy?

Philip Pullman's series follows Lyra Belacqua across parallel worlds.

Dark matter serves as a plot device in Soft Light, a 1995 episode of which series?

In fiction the substance is usually given powers it could never have in physics.

Which Dutch astronomer hypothesised dark matter in 1932 from stellar motions near the Sun?

He later wrote about the large non-visible halo of the galaxy NGC 3115.

Which galaxy did H. W. Babcock report a puzzling rotation curve for in 1939?

He blamed light absorption or modified dynamics rather than unseen matter; the object was then called a nebula.

What kind of signal do direct-detection experiments look for when a WIMP hits an atomic nucleus?

The recoil shows up as scintillation light, ionisation or heat.

Roughly how many dark matter particles might pass through each square centimetre of Earth every second?

Yet none has ever been caught in a detector.

In which year did Vera Rubin die?

She died of natural causes at 88 on Christmas night.

Which Swedish astronomer's 1930 paper first hypothesised the universe holds far more mass than is seen?

Jacobus Kapteyn had suggested dark matter from stellar velocities in 1922.

Unlike modern theories, what did Zwicky himself think the missing mass was?

He simply assumed the missing mass was ordinary matter that did not shine.

Zwicky emigrated to the United States in 1925 to work with which physicist at Caltech?

He came on a Rockefeller Foundation fellowship and became professor of astronomy in 1942.

Albert Bosma's 1978 evidence for flat rotation curves came from which Dutch instrument?

Interferometric arrays let astronomers trace hydrogen far beyond the visible disk.

Which balloon-borne experiment discovered the first acoustic peak in the CMB in 2000?

COBE had found the anisotropies in 1992 but was too coarse to see peaks; WMAP and Planck later mapped the spectrum precisely.

Galaxy pairs show a subtle preference for what separation, the baryon acoustic oscillation scale?

The roughly 1% excess was predicted in the 1990s and found in 2005 by the Sloan and 2dF redshift surveys.

Big Bang nucleosynthesis limits ordinary baryonic matter to what share of the critical density?

Total matter is about 30% of critical density, so most of it cannot be made of protons and neutrons.

Who first proposed primordial black holes in 1966, before Hawking did so independently in 1971?

Interest revived after LIGO's 2015 detection of merging 30-solar-mass black holes, hard to explain via ordinary stellar collapse.

What November 2025 LIGO/Virgo/KAGRA report boosted the primordial black hole hypothesis?

No known astrophysical process makes black holes below the 1.4-solar-mass Chandrasekhar limit.

What are JWST's compact, very high-redshift objects that hint at 'heavy seed' black holes called?

Galaxies such as UHZ1 and GHZ2 hold supermassive black holes less than 500 million years after the Big Bang.

Which collaboration claims to see an annual modulation in its detector as Earth moves through the halo?

The claim remains in tension with null results from more sensitive noble-liquid and cryogenic experiments.

Directional detectors seek WIMP signals aligned with the Solar System's motion towards which constellation?

Experiments such as DRIFT, DMTPC and MIMAC use low-pressure time projection chambers to try to see this 'WIMP wind'.

Which probe's electron-positron data helps rule out tiny evaporating black holes as all dark matter?

Microlensing data from the Subaru telescope's Hyper Suprime-Cam closes other mass windows, though broad mass distributions loosen the limits.