60 Fun Facts About Dark Matter

Why are dark matter detectors such as LZ built deep underground?

To shield them from cosmic rays. Under Italy's Gran Sasso mountain, 1,400 metres of rock cut that rain of particles about a million times, so a single rare collision has a chance of standing out.

Dark matter outweighs ordinary matter, but what makes up the largest share of the universe?

Dark energy. It makes up roughly two-thirds of the universe, so dark matter, though it outweighs every star, planet and gas cloud put together, is only the runner-up.

Dark matter outweighs ordinary matter roughly how many times over?

5 times. Dark matter is about 85% of all matter, which is why the stars and gas we can see supply far too little gravity to hold a spinning galaxy together.

Dark matter is spread so thinly near the Sun that all of it inside Neptune's orbit weighs about as much as what?

A large asteroid. Dark matter outweighs ordinary matter only because it fills the vast spaces of the galaxy; near the Sun it is spread far more thinly than normal matter.

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

Lord Kelvin. He treated the Milky Way's stars like the particles of a gas, worked out from their speeds how much mass could be present, and concluded that much of it might not shine.

Which French scientist wrote of 'matière obscure', or dark matter, in 1906?

Henri Poincaré. He raised the idea only to play it down, reasoning that unseen stars could not outweigh the shining ones, a conclusion later measurements overturned.

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

Coma. Its galaxies were moving so fast that Zwicky reckoned the cluster needed some 400 times more matter than he could see to keep them from flying apart.

Why did Fritz Zwicky's 1930s estimates overstate the ratio of unseen to visible matter?

He used an obsolete Hubble constant. The 1930s value was about eight times today's, and correcting it shrinks his ratio by roughly that much, yet the cluster still needs far more mass than its stars supply.

In which animated comedy is dark matter a starship fuel, collected from the droppings of small aliens?

Futurama. In the film Bender's Game the tycoon Mom corners the supply, and her dark matter 'mine' turns out to be a farm of captive Nibblonians, the species of the crew's pet Nibbler.

Which astronomical term did Fritz Zwicky help coin in the 1930s while proposing neutron stars?

Supernova. Zwicky and his colleague Walter Baade began using the word in their lectures, then Zwicky set out to find the explosions himself and discovered dozens in other galaxies.

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

Spherical bastards. Zwicky explained the geometry: like a sphere, they looked exactly the same, bastards, from whichever angle you viewed them.

The LZ dark matter detector sits about a mile underground in South Dakota, inside a former what?

Gold mine. The Homestake mine had been worked since the Black Hills gold rush of 1876, and an experiment deep inside it was catching particles from the Sun in the 1960s, decades before the whole mine became a laboratory.

What did Vera Rubin find at spiral galaxies' outer edges?

They move as fast as material near the centre. Planets far from the Sun crawl along their orbits, so stars at a galaxy's rim should crawl too, unless unseen mass is pulling them along.

Which famous observatory, lacking a women's restroom, gave Vera Rubin permission to observe in 1965?

Palomar. Warned that her time was limited because there was no ladies' room, Rubin cut a skirt out of paper, stuck it on the figure on the men's room door and got to work.

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

George Gamow. The thesis she wrote for him concluded that galaxies clump together instead of lying scattered at random, a finding that went largely unrecognised until the 1970s.

Which honour was Vera Rubin frequently tipped for but never awarded?

The Nobel Prize. It cannot be awarded posthumously, so Rubin's chance ended when she died at 88, after years of being named as a candidate.

The Vera C. Rubin Observatory stands on a mountaintop in which country?

Chile. From its Andean peak it photographs the entire southern sky every few nights, and over ten years the frames will add up to a time-lapse film of the changing universe.

What record did the Rubin Observatory's main instrument set?

It is the largest camera ever built. The size of a small car, it takes in about 45 full Moons of sky in one exposure, and a single image would need hundreds of ultra-HD screens to show in full.

Which European space telescope was launched in 2023 to map dark matter across a third of the sky?

Euclid. It is named after the Greek founder of geometry, because the amount of matter and energy in the universe decides the geometry of space itself.

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

The 21 cm line of atomic hydrogen. Hydrogen gas stretches far past a galaxy's last stars, so its radio signal let astronomers clock the rotation where there was nothing else to see.

Which galaxy cluster shows most of its mass sitting apart from its hot gas, as direct evidence for dark matter?

The Bullet Cluster. One cluster punched through another there: drag slowed the two clouds of hot gas like air resistance, while the dark matter, feeling only gravity, carried on ahead with the galaxies.

Colliding galaxy clusters, where mass and hot gas part company, challenge which type of theory?

Modified gravity. If there were no unseen matter and gravity simply worked differently, the argument goes, the strongest pull should sit on the hot gas, which holds most of the ordinary matter, not off to one side of it.

Astronomers weigh a galaxy cluster from its gas, which at tens of millions of degrees glows in what kind of light?

X-rays. Three independent ways of weighing a cluster, by its galaxies' speeds, its X-ray gas and the light it bends, agree that most of its mass cannot be seen.

In 'cold dark matter', what does 'cold' mean?

Its particles moved slowly in the early cosmos. Slow particles could start clumping at once, so small structures formed first and merged into ever bigger ones, the pattern surveys of galaxies show.

Which known particles were ruled out as the main dark matter because they move too fast to clump into galaxies?

Neutrinos. Particles that fast would have built vast sheets the size of superclusters first and only later broken them into galaxies, but surveys show the galaxies came first.

What does the acronym WIMP stand for?

Weakly interacting massive particle. 'Weakly' points at the weak nuclear force: besides gravity, nothing stronger would link such a particle to ordinary matter, so it could pass through the whole Earth unnoticed.

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

Liquid xenon. LZ holds seven tonnes of it: when a particle strikes one of its atoms the liquid gives off a faint glimmer, and hundreds of light sensors above and below are waiting to catch it.

What is the 'neutrino fog' that hampers dark matter detectors?

An irreducible background of solar neutrino signals. No shield stops neutrinos from the Sun, and the rare trace one leaves in the tank looks just like the dark matter particle the detector is waiting for.

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

Supersymmetry. It gives every known particle a heavier partner, and the lightest of those partners would have made an ideal WIMP, but years of collisions have not produced a single one.

Which dark matter candidate did Frank Wilczek name after a brand of laundry detergent?

The axion. Wilczek had spotted Axion detergent in a supermarket and thought it sounded like a particle, so he used the name for one that 'cleaned up' a problem; a rival had wanted to call it the higglet.

The axion was invented to explain why which force of nature treats matter and antimatter so evenly?

The strong force. Its equations allow it to favour matter over mirror-image antimatter, yet no experiment has ever caught it doing so; the proposed particle's field would cancel the bias automatically.

The ADMX experiment uses a powerful magnet to try to convert axions into which particles?

Photons. ADMX works like a radio receiver: a tunable cavity sits inside the magnet, and physicists slowly adjust it, hoping to hit the one frequency set by the particle's unknown mass.

What does MACHO stand for?

Massive compact halo object. Astrophysicist Kim Griest coined the name as a deliberate counterpart to the rival WIMPs; it covers dim, heavy bodies such as brown dwarfs and lone planets.

The Alpha Magnetic Spectrometer, which hunts for antimatter made by dark matter collisions, is bolted to which spacecraft?

The International Space Station. Fixed to the station's frame since 2011, it has found a surplus of antimatter positrons that could be debris from dark matter collisions, though other causes are not ruled out.

MOND, a rival idea to dark matter, rewrites whose laws at very low accelerations?

Isaac Newton. Israeli physicist Mordehai Milgrom proposed in 1983 that gravity fades more slowly than Newton's law says once accelerations become tiny, which would explain how galaxies rotate with no hidden mass.

What sign would betray an unseen dark matter particle made in the Large Hadron Collider?

Missing energy. A dark matter particle would slip out through the detector without a trace, so physicists add up everything a collision produced and look for a shortfall.

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

Driving the universe's accelerating expansion. Astronomers expected gravity to be slowing the expansion down, until in 1998 distant exploding stars turned out dimmer, and so farther away, than any slowing universe allowed.

What role is dark matter thought to play in galaxy formation?

Gravitational scaffolding for cosmic structures. Dark matter clumped first, into a web of filaments, and ordinary gas then fell into the densest knots, where it lit up as galaxies.

Galaxy maps use which measurement as a stand-in for distance, making superclusters look squashed along our line of sight?

Redshift. A supercluster's gravity pulls the galaxies around it inward, which shifts their redshifts and flattens the structure on the map; the size of the squash shows how much mass is there.

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

Arcs. The cluster's mass, most of it dark, bends the light of galaxies far behind it like a flawed lens, smearing them into curved streaks whose shapes let astronomers map the unseen matter.

In Philip Pullman's His Dark Materials, which mysterious substance is the counterpart of dark matter?

Dust. A physicist in the story is studying dark matter when she learns that her particles and Dust are one and the same, the stuff the Church in Lyra's world fears as the trace of original sin.

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

The X-Files. A physicist's accident with dark matter turns his shadow lethal, reducing anyone it touches to a scorch mark; the script was the first the show bought from a fan named Vince Gilligan, later the creator of Breaking Bad.

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

Jan Oort. He judged that the stars he tracked needed more mass in the galaxy's disc than astronomers could see, though his measurement was later found to be wrong.

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

Andromeda. Babcock put the odd result down to light being absorbed inside the galaxy or to unusual dynamics at its rim, never to unseen matter, a link others made only decades later.

What kind of signal do direct-detection experiments hope a passing WIMP will leave?

A low-energy nuclear recoil. A WIMP would knock the core of a single atom like one billiard ball striking another, and the jolted atom gives itself away as a glimmer of light, freed electric charge or heat.

In 2007 astronomers unveiled the first 3D map of dark matter, built from a survey by which space telescope?

The Hubble Space Telescope. In the largest survey it had then undertaken, a patch of sky nine times the full Moon's area, astronomers measured how unseen mass had warped the shapes of distant galaxies behind it.

Which NASA rover spent more than a year exploring a ridge on Mars named after Vera Rubin?

Curiosity. It climbed onto Vera Rubin Ridge, on the flank of Mount Sharp, in September 2017 and drilled into its rocks until early 2019 before driving on to the clay beds below.

NASA's Fermi space telescope searches for dark matter by watching for which kind of light from colliding particles?

Gamma rays. If dark matter particles collide and destroy one another, their energy should emerge as gamma rays, and Fermi has found an unexplained surplus at the Milky Way's centre, though pulsars could be the cause.

What did Fritz Zwicky himself think the missing mass was?

Non-luminous ordinary matter. Zwicky pictured cool, faint stars, solid bodies and gas that simply did not shine; the idea that it might be an unknown kind of particle arrived decades later.

Fritz Zwicky left Switzerland in 1925 for which American university, where he worked for the rest of his career?

Caltech. He arrived on a Rockefeller fellowship to work with the physicist Robert Millikan and, proud of being Swiss, never took American citizenship.

In which superhero TV series does a wave of dark matter turn a busload of passengers into 'metahumans'?

The Flash. Pulling the hero out of the Speed Force drenches a city bus in dark matter, and that season's villain, the Thinker, then hunts the twelve new metahumans to steal their powers.

The BOOMERanG telescope mapped the Big Bang's afterglow while being carried around Antarctica by what?

A high-altitude balloon. Winds that circle the South Pole carried it right around Antarctica and back within two weeks, and its map of hot and cold spots showed that the universe's geometry is close to flat.

Galaxies slightly prefer one particular spacing, the frozen imprint of which waves in the early universe?

Sound waves. Pressure waves rang through the hot gas of the infant universe until it cooled and they froze in place, leaving galaxies slightly more likely to sit nearly 500 million light-years apart.

The Big Bang's first minutes forged hydrogen, helium and traces of which other element?

Lithium. The measured mix of these light elements fixes ordinary matter at about a twentieth of the universe, far too little to supply the gravity astronomers see, so most dark matter cannot be made of atoms.

Which British physicist argued in the early 1970s that small black holes could have formed in the Big Bang itself?

Stephen Hawking. Two Soviet theorists, Yakov Zeldovich and Igor Novikov, had raised the idea in 1966, and such 'primordial' black holes are now studied as one possible form of dark matter.

Which November 2025 LIGO/Virgo/KAGRA alert, if real, could point to black holes older than any star?

A candidate sub-solar-mass merger. A dying star cannot leave a black hole lighter than the Sun, so one that small would have to be a relic of the Big Bang, though the signal may yet prove to be noise.

What nickname do astronomers use for the compact objects JWST found crowding the early universe, many thought to hide growing black holes?

Little Red Dots. They crowd the universe's first billion and a half years and then all but vanish, with almost nothing like them found in the nearby cosmos.

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

DAMA/LIBRA. Its signal has never been reproduced: COSINE-100 and ANAIS-112, built from the same sodium iodide crystals to test the claim, found no yearly rise and fall.

The Sun's orbit round the galaxy carries us towards which constellation, where directional detectors look for a WIMP 'wind'?

Cygnus. Dark matter should seem to stream from near its bright star Deneb, a direction fixed on the sky that no known background on Earth or in space can mimic.

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

Voyager 1. Evaporating black holes should spray out low-energy electrons and positrons that the Sun's magnetic field keeps away from Earth, so only a probe beyond the heliopause can count them.