Main Page

Explain xkcd: It's 'cause you're dumb.
Revision as of 17:52, 5 June 2018 by Dgbrt (talk | contribs) (New here?: Link to the Editor FAQ)
(diff) ← Older revision | Latest revision (diff) | Newer revision → (diff)
Jump to: navigation, search

Welcome to the explain xkcd wiki!
We have an explanation for all 2034 xkcd comics, and only 20 (1%) are incomplete. Help us finish them!

Latest comic

Go to this comic explanation

Dark Matter Candidates
My theory is that dark matter is actually just a thin patina of grime covering the whole universe, and we don't notice it because we haven't thoroughly cleaned the place in eons.
Title text: My theory is that dark matter is actually just a thin patina of grime covering the whole universe, and we don't notice it because we haven't thoroughly cleaned the place in eons.


Ambox notice.png This explanation may be incomplete or incorrect: Every section needs to be filled and explained. Do NOT delete this tag too soon.

Dark matter is a hypothetical 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 sum of the masses of visible matter 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. 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 possibly be, charted by mass from smallest (given in electronvolts) to largest (given in kilograms). Masses in the range 10-15 kg to 10-3 kg are given in grams together with appropriate prefixes, while the ton takes the place of 103 kg.

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.


An axion is a hypothetical elementary particle that might be a component of dark matter.

Sterile neutrino

Sterile neutrinos are hypothetical particles interacting only via gravity. It's an actual candidate for dark matter.

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, 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.


A neutralino is a hypothetical particle from supersymmetry, it is an actual candidate for dark matter.


In theoretical physics, a Q-ball is a stable group of particles. 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 explainxkcd uses for the most common xkcd character.)


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 are a family (Ceratopogonidae) of small flies, 1–4 mm long, that can pass through most window screens. Another joke candidate.


Insects of the clade Antophila 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.


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.

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 space western TV show, 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".

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. 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 sun 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 sun masses). But recent gravitational wave detections indicate that black holes at 50 or 100 sun masses also exist, though their origin is still not understood. Randall doesn't mention this but some astronomers hope that these could fill at least a part of the gap. Regardless, they have been ruled out as a candidate for dark matter for various reasons Randall has listed.

Except the last item, all range below the mass of the sun (2x1030 kg) while the smallest known black hole is about four sun masses.

  • Gamma rays: If dark matter were black holes of this size, the black holes would be evaporating in bursts of Hawking radiation, and we'd see a buzz of gamma rays from every direction.
  • 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, (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 kg - 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 would be impossible to miss, due to the effects they have on nearby matter. But at the mass of some 1030 kg there must be many supernova remnants we still haven't found.
Maybe those orbit lines on space diagrams are real and very heavy

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, though. 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 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 would be a problem. 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. Another joke candidate.

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 inside there in stars and clouds -- the space between is even more empty. For comparison, one gram hydrogen consists of 6.022 x 1023 atoms. Like at home wiping with a cleaning cloth 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 cubic light years. Atoms can't hide; there is always radiation.


Ambox notice.png This transcript is incomplete. Please help editing it! Thanks.
Dark matter candidates:
[A line graph is shown and labeled at left quarter in eV and further to the right in g together with some prefixes.]
[The labels read:]
µeV, meV, eV, keV, MeV, GeV, TeV, 10-18kg, ng, µg, mg, g, kg, TON, 106kg, 1012kg, 1018kg, 1024kg, 1030kg
[All items are shown in bars ranging between two approximately values:]
< 1 µeV - 10 meV: Axion
1 eV - 10 keV: Sterile neutrino
0.5 MeV (exactly): Electrons painted with space camouflage
10 GeV - 10 TeV: Neutralino
100 TeV - 10-17 kg: Q-ball
1 ng - 100 ng: Pollen
0.1 mg - 1 mg: No-See-Ums
10-1 g (exactly): Bees
10 g - 100 g: 8-balls
100 kg - TON: Space cows
TON - 109 kg: Obelisks, monoliths, pyramids
109 kg - 1033 kg: Black holes ruled out by:
109 kg - 1013 kg: Gamma rays
1013 kg - 1017 kg: GRB lensing
1015 kg - 1022 kg: Neutron star data
1021 kg - 1030 kg: Micro lensing
1024 kg - 1030 kg: Solar system stability
1030 kg - 1033 kg: Buzzkill astronomers
1033 kg - >1036 kg: Maybe those orbit lines on space diagrams are real and very heavy

Is this out of date? Clicking here will fix that.

New here?

Last 7 days (Top 10)

Lots of people contribute to make this wiki a success. Many of the recent contributors, listed above, have just joined. You can do it too! Create your account here.

You can read a brief introduction about this wiki at explain xkcd. Feel free to sign up for an account and contribute to the wiki! We need explanations for comics, characters, themes and everything in between. If it is referenced in an xkcd web comic, it should be here.

  • There are incomplete explanations listed here. Feel free to help out by expanding them!
  • We sell advertising space to pay for our server costs. To learn more, go here.


Don't be a jerk.

There are a lot of comics that don't have set-in-stone explanations; feel free to put multiple interpretations in the wiki page for each comic.

If you want to talk about a specific comic, use its discussion page.

Please only submit material directly related to (and helping everyone better understand) xkcd... and of course only submit material that can legally be posted (and freely edited). Off-topic or other inappropriate content is subject to removal or modification at admin discretion, and users who repeatedly post such content will be blocked.

If you need assistance from an admin, post a message to the Admin requests board.