xkcd.WTF!?

Explanation ➲

Dark matter is a hypothetical, invisible form of matter used by the vast majority of astronomers to explain the far-too-high apparent mass of objects at large scales in our universe. In galaxies, stars are orbiting faster than the gravitational force of the visible matter (stars, gas, dust, etc.) in the galaxy could cause, and entire galaxies are observed moving much faster around each other than their visible masses could explain. In galactic collisions, the mass can appear to separate from the visible matter, as if the mass doesn't collide but the visible matter does. A small handful of galaxies have been observed to not have this property, suggesting that it is a thing that a galaxy can have more or less of and is separable from. The most plausible explanation for all of these phenomena is that there is some matter present that doesn't emit light - "dark matter" - and that has gravity, but is otherwise undetectable. At scales of our solar system, those effects are too small and can't be measured. In cosmology, dark matter is estimated to account for 85% of the total matter in the universe.

This comic gives a set of possibilities for what dark matter could be, charted by mass from smallest (given in electronvolts) to largest (given in kilograms). Masses in the range 10⁻¹⁵ to 10⁻³ kg are given in grams together with appropriate prefixes, while the ton takes the place of 10³ kg.

Only massive objects ranging from subatomic particles up to supermassive ones are covered in this comic. There are also alternative hypotheses trying to modify general relativity with no need of additional matter. The problem with these theories is that they can't explain other existing observations. Nonetheless dark matter is a mystery because no serious candidate has been found yet.

The joke in this comic is that the range of the mass of the possible particles and objects stretch over 81 powers of ten, with explanations suggested by astronomers covering only some portions of that range. Randall fills the gaps with highly absurd suggestions.

Axion

An axion is a hypothetical elementary particle postulated in 1977 to resolve the strong CP problem in quantum chromodynamics, a theory of the strong force between quarks and gluons which form hadrons like protons or neutrons. If axions exist within a specific range of mass they might be a component of dark matter. The advantage of this particle is that it's based on a theory which could be proved or also disproved by measurements in the future. Other theories not mentioned in this comic, like weakly interacting massive particles (WIMPs), are much more vague.

Sterile neutrino

Sterile neutrinos, an actual candidate for dark matter, are hypothetical particles interacting only via gravity. Normal non-sterile neutrinos are charged under the weak interaction and can be detected by experiments using that interaction.

Electrons painted with space camouflage

Electrons are fundamental particles which compose the outer layers of atoms. A large number of electrons in the galaxy would be relatively easy to detect, as they not only interact with light (which dark matter does not appear to), but also have a strong electric charge. Presumably, space camouflage is a positively-charged coating which prevents electrons from interacting with light. (Needless to say,^{[citation needed]} this is not an actual candidate for dark matter.) The mass of an electron is about 0.5 MeV, which fits well into the graph.

Neutralino

A neutralino is a hypothetical particle from supersymmetry and is also a current candidate for dark matter. However, there is no evidence at present whether or not supersymmetry is correct, and none of its predicted particles have been found yet.

Q-ball

In theoretical physics, a Q-ball is a stable group of bosons. It's an actual candidate for dark matter. (In billiards, a cue ball is the white or yellow ball hit with the cue in normal play. In addition, Cueball is the name explain xkcd uses for the most common xkcd character.)

Pollen

Pollen is a joke candidate, though people with seasonal allergies may suspect that the universe is genuinely made up entirely of pollen in the springtime.

No-see-ums

No-see-ums are a family (Ceratopogonidae) of small flies, 1–4 mm long, that can pass through most window screens. Another joke candidate, because dark matter is invisible and the name "no-see-ums" implies that the flies are invisible.

Bees

Insects of the clade Anthophila are major pollinators of flowering plants. In recent years bees have been disappearing at an alarming rate; Doctor Who explained that they are in fact aliens leaving Earth prior to a Dalek invasion.

8-balls

In pool, the 8-ball is a black ball numbered 8. It's a pun with Q-ball/cue ball. Unless undetected aliens have discovered billiards and become addicted to it, 8-balls are found only on Earth and are, hence, unlikely dark matter candidates. The 8-ball is also a popular unit of sale for black market pharmaceuticals like cocaine, where it stands for ⅛ ounce (3.5 g). This doesn't make sense as a dark matter candidate either – unless dark matter is hard to detect because it's illegal and trying to avoid the police.

Space cows

Cows are bovines extensively farmed on Earth for milk and meat. Although there is folklore concerning cows achieving circum-lunar orbits, not to mention their appearance on a beloved space western TV show, as Muppet cow Natalie in the Sesame Street News Flash (and others less-remembered), they have yet to be found elsewhere in the universe. In the television show "Too Close for Comfort", one of the characters is the cartoonist of a comic strip called "Cosmic Cow". Spherical cows (and especially those in a vacuum, as they would essentially be if in space) have also been used (humorously) by physicists needing to simplify some source of mass in a given problem.

Obelisks, monoliths, pyramids

While those human constructions are huge on a human scale, they're negligible at universe-scale. It would take a large number of such constructions, distributed through space, to replicate the effects of dark matter; while a scenario could be envisioned where enough such constructs existed, with properties and distribution allowing them to match observations, this is obviously not a likely explanation.^{[citation needed]} They often show up in fiction and pseudo-scientific literature as alien artifacts generating immense unknown power out of nowhere, with the most famous and influential example being the three monoliths from 2001: A Space Odyssey (with the largest having a mass of about 500,000 tonnes).

Black holes ruled out by

Black holes are known to occur in sizes of a few solar masses (about 10³⁰-10³¹ kg) as remnants of the core of former big stars, as well as in quite large sizes at the centers of galaxies (millions or even billions of solar masses). Black holes of various sizes have been proposed as dark matter candidates, though to date they have mostly been ruled out for the various reasons Randall lists here; that this is a common practice in cosmology may be part of the reference to "buzzkill" astronomers. (However, such constraints are based on "monochromatic" mass distributions -- meaning that all such black holes are assumed to have the same mass -- which is considered physically implausible for populations of merging bodies which are known to have vastly different masses. See Primordial black holes as dark matter (2017) and Primordial black hole constraints for extended mass functions (2017).) Randall rules out all black holes in the range of approximately 10¹⁰ kg to 10³³ kg, even when there are some gaps between his bars.

Except for the last item, all of these are below the mass of the sun (2×10³⁰ kg), while the smallest known black hole is about four solar masses. Nevertheless smaller black holes have been proposed as dark matter candidates.

• Gamma rays: If dark matter were black holes of this size, the black holes could be evaporating by the predicted Hawking radiation, and we'd see a buzz of gamma rays from every direction if many of those objects would exist. This radiation is still hypothetical and has not been observed from any known black holes. Furthermore those objects would be very small, because the Schwarzschild radius of a 10¹² kg black hole is approximately 148 fm (1.48×10⁻¹³ m), between the size of an atom and an atomic nucleus.

• GRB lensing: Gamma-ray bursts (GRBs) are the brightest events in the universe and have been observed only in distant galaxies. While gravitational microlensing (see below) is an astronomical phenomenon, it doesn't make much sense here. GRBs are short (milliseconds to several hours) and are often detected only by space-borne sensors for gamma-rays -- rarely at any other wavelengths. Measuring lensing effects would be very difficult. This paper discusses the probability of detecting lensing effects caused by galactic halo objects among the known GRBs given sufficient objects to represent the missing mass.

• Neutron star data: Neutron stars aren't black holes, but they're also very small highly compact objects at about 1.4-2.16 solar masses. While black holes can't be observed directly, neutron stars are detectable in many wavelengths. The number of them gives a clue about the number of black holes close to the mass of the sun, a number which is far too low to make up dark matter.

• Micro lensing: Gravitational microlensing is a gravitational lens effect in which the path of radiation is changed by passing through space bent by nearby mass. This was predicted by Einstein's Theory of General Relativity and was first confirmed in 1919 during a solar eclipse, when a star which was nearly in line with the sun appeared more distant to the sun than usual. Astronomers have found many so called Einstein rings or Einstein crosses where a massive object in front of other galaxies bends the light toward us. Those massive objects may be black holes, but the number is far too low to explain dark matter.

• Solar system stability: Our solar system is 4.5 billion years old and has been very stable since shortly after its formation. If not, we wouldn't exist. If dark objects at 10²⁴ to 10³⁰ kg (mass of Earth up to mass of Sun) accounted for dark matter and were distributed throughout galaxies, there should be many of them in the vicinity of our solar system and the system wouldn't be stable at all.

• Buzzkill astronomers: Black holes above a certain size are thought by some astronomers to be impossible to miss, due to the effects they have on nearby matter. At the mass of some 10³⁰ kg there must be many supernova remnants we still haven't found. Black holes of about 10³⁵ kg have long been considered dark matter candidates by a minority group of cosmologists, as could be seen here Primordial Black Holes as All Dark Matter (2010) and the Milky Way's first discovered intermediate mass black hole falling in this range shown here Signs of Second Largest Black Hole in the Milky Way.

Not covered by this comic are massive astrophysical compact halo objects (MACHOs) composed of hard-to-detect dim objects like black holes, neutron stars, brown dwarfs, and other objects composed of normal baryonic matter. Nevertheless observations have shown that the total amount of baryonic matter in our universe on large scales is much smaller than it would be needed to explain all the measured gravitational effects.

Maybe those orbit lines on space diagrams are real and very heavy

Another joke candidate. Diagrams of our solar system (or any planetary system) often show lines representing the elliptical paths the planet takes around its sun. These lines don't show real objects;^{[citation needed]} astronomers just draw them on pictures of the solar system to show where the planets move. If you draw a line on a map to give someone directions, that line isn't an object in real life; it's just some ink on the map. If these lines were real, they would be huge (Earth's would be 940 million km long (2π AU) and Neptune's would be 28 billion kilometers long). Powers of Ten (1977) gives a good sense of just how large these orbit lines need to be in order to be visible in space diagrams. If these orbit lines were also very dense, they would have a huge mass and could possibly account for the missing 85% of the mass in the universe. But they would also constantly be impaling the planets, including the Earth, which might be a problem.^{[citation needed]} Their mass would also affect planetary motions in ways which we would detect. A related worry about space travel was expressed in previous centuries; it was thought that the planets were embedded within crystal shells (spheres or Platonic solids), and a rocket into space could smash the shells and send planets plummeting to Earth.

Title text

The title text refers to the fact that space is just vast emptiness where a little bit of dirt could be overlooked. Actually the mean density of detectable matter in the universe, according to NASA, is equivalent to roughly 1 proton per 4 cubic meters. And because this matter is mostly located in galaxies -- and within them, in stars and clouds -- the space between is even more empty. For comparison, one gram of hydrogen consists of 6.022 × 10²³ atoms. Like wiping with a cleaning cloth at home in which we can see the dirt that wasn't clearly visible on the surface we have wiped, Randall believes that some few atoms more per cubic meter could stay undetected in the same way. This isn't true because in the space between galaxies, astronomers can detect matter as it spreads over thousands or millions of cubic light years. Atoms can't hide; there is always radiation.