2035: Dark Matter Candidates

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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.
If you can address this issue, please edit the page! Thanks.
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 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 of 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 grammes.

The joke in this comic is that the range of the mass of the possible particles and objects stretch over 81 powers of ten. Randall filled the gap between real small candidate particles and real large candidate objects 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 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.)


A Neutralino is a hypothetical particle from Supersymmetry, not something made up by Randall Munroe that sounds vaguely like one. It's 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.


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


No-See-Ums, also called Ceratopogonidae, a family of small flies (1–4 mm long) who can pass through most window screens. Another joke candidate.


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 folk lore concerning cows acheiving circum-lunar orbits, they have yet to be found elsewhere in the Universe.

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 monolith from 2001: A Space Odyssey.

Black Holes ruled out by

Black holes are known in sizes of a few sun masses (about 1030-1031 kg) as remnants of the core of former big stars and the real big ones at the centers of galaxies (millions or even billions of the mass of the sun.) But recent gravitational wave detection indicate that black holes at 50 or 100 sun masses also exist while 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.

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
  • Neutron Star Data
  • Micro lensing
  • Solar System Stability
  • Buzzkill Astronomers: Black holes above a certain size would be impossible to miss, due to the effects they have on nearby matter.
Maybe those orbit lines on space diagrams are real and very heavy

Any diagram of our solar system (or any solar system) will show lines representing the path the planet takes around its sun. Since planets orbit in ellipses, there will be an ellipse for every planet. This 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 inner four planets, including the Earth, which would be a problem. Overall, not a very likely candidate.


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
1 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

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"thin patina of grime covering the whole universe" is a reference to the "International prototype kilogram" and the necessity to keep it dust-free to preserve its reference status. 11:14, 20 August 2018 (UTC)

I think it's just referring to how your room or furniture can get super dirty and completely covered in dust, but you don't really notice it getting dirty because it happens so gradually. But once you actually get around to cleaning your room and you remove all the dust you realize how insanely filthy your room was, now that you can compare it to clean. Since there hasn't been a massive universe cleaning within human history, we wouldn't really be able to tell if the universe was coated in dirt because we wouldn't remember what it looks like clean. Yosho27 (talk) 12:53, 20 August 2018 (UTC)
I concur, my thought upon reading the "thin patina of grime" was when I helped a friend power wash his back deck and we realized it was far more dirty than we thought; as the newly washed sections stood out in stark contrast to the grimy parts. 19:29, 20 August 2018 (UTC)

One can only hope that the solution for dealing with space cows involves space cowboys. 20:40, 20 August 2018 (UTC)

109 kg - 1033 kg black holes

Not sure if it's a mistake by Randall or he has something other in mind. But most of his black holes are far too lightweight:

  • 109 kg is a million tons, the Great Pyramid of Giza wights six times of that
  • 6x1024 kg Earth
  • 2x1030 kg Sun
  • 1031 kg smallest known stellar black hole
  • 1040 kg the real big black holes with a diameter in the size of our solar system

Everything except the Buzzkill is below a single solar mass. --Dgbrt (talk) 16:24, 20 August 2018 (UTC)

The theoretical lower limit for black hole mass is the planck mass (22 µg), although such micro black holes would evaporate very quickly under standard models. However, larger black holes were excluded fairly early by gravitational lensing searches ('buzzkill' cases), so smaller black holes had to be considered separately as dark matter candidates. --Quantum7 (talk) 20:40, 20 August 2018 (UTC)
You misunderstand my point: Those not discovered smaller black holes would need an explanation how they did form but more important here how they could be ruled out as Randall states. A nano black hole at 1010 kg disproved by gamma rays? What's Randall's point? He was more accurate in the past. --Dgbrt (talk) 22:18, 20 August 2018 (UTC)
Um, his point is that we know that black holes that size (regardless of how they came into existence) would "evaporate" in a burst of gamma rays through the process that causes Hawking radiation. Which the explanation above, you know, explains. Similarly, other light black holes (which would be formed by any number of theoretical processes other than collapsing stars, usually involving conditions early in the Big Bang) would be ruled out by the other reasons given, also explained in the explanation. (talk) (please sign your comments with ~~~~)
Axon pun?

My first thought upon reading 'axion' was that it was a pun on axon. Neurons have typical membrane potentials in the mV range, which lines up nicely with the meV energy of axions. Coincidence? --Quantum7 (talk) 20:44, 20 August 2018 (UTC)

An axion is a suggested subatomic particle. I'm not a biologist but if one meV is enough energy to trigger an axon our biology wouldn't work that smoothly. --Dgbrt (talk) 22:18, 20 August 2018 (UTC)
It's mV (electrical potential), not meV (energy/mass). It's a stretch, but Randall's included more distant puns before in XKCD. Source for action potential strength: [1] --Quantum7 (talk) 23:15, 20 August 2018 (UTC)

While this comic is about Dark Matter, does the explanation really need to include a justification on why Dark Matter really exists as a "substance" instead of being some error in our understanding of gravity? It seems a little excessive and unnecessary to me. Ianrbibtitlht (talk) 21:46, 20 August 2018 (UTC)

I'm with you but this comic is about that "substance" like most astronomers are. This always reminds me to aether - also a famous "substance" in space more than hundred years ago which nobody could explain. --Dgbrt (talk) 22:32, 20 August 2018 (UTC)
Thanks for the laugh - my thoughts exactly! In fact, part of me wonders if Randall is actually making fun of the whole idea that there's a dark matter particle at all, since there's such a wide range of possible sizes for such a particle. His humor can be so subtle at times that someone may not realize when they're actually the butt of his joke. Ianrbibtitlht (talk) 23:55, 20 August 2018 (UTC)
Actually, the observings that dark matter doesn't seem to be at same places as normal matter is countering the idea that it's because of error in out understanding of gravity. Like, not completely disproving it, but making it less likely. -- Hkmaly (talk) 04:15, 21 August 2018 (UTC)
Excessive? Maybe. But the first responses to you indicate that people who have presumably even read the explanation as to why dark matter really exists don't understand why we expect that dark matter really exists. (Sure, modified gravity theories were a reasonable alternative hypothesis fifteen years ago, but that was before we'd made multiple independent observations that the gravitational effects are decoupled from the presence of visible matter, and thus cannot simply be gravity working differently at galactic mass scales than General Relativity predicts.) (talk) (please sign your comments with ~~~~)
I'm not interested in debating which viewpoint is correct. I'm not even picking a side, and yet others seem eager to argue their side with me. I'm only asking if that even needs to be included in the explanation, as it tends to distract from the points made in the comic. I think it might be more helpful to mention why it's called dark matter in the first place, which I don't see at all - maybe because of this distraction. Please remember that our primary purpose is to explain the comic, not to write a wikipedia article on the subject matter. Thanks for sharing though. Ianrbibtitlht (talk) 05:31, 21 August 2018 (UTC)

Furthermore, while space cowboys were mentioned earlier in the discussion, I suspect Randall included space cows in the chart specifically as a reference to the movie Space Cowboys. Also, I think the point about Neutron Star Data ruling out black holes in that mass range is because you can't have both of them with the same mass, since the current theory is that they both form from a star collapse, but at different masses. You're always going to get one or the other from that size star, and since we find neutron stars in that range, we can't have black holes there too. Ianrbibtitlht (talk) 21:59, 20 August 2018 (UTC)

The mass of neutron stars is well understood. A smaller star ends at a white dwarf and the big ones produce a black hole. Nonetheless our sun will end up into a white dwarf and the others require higher masses as in the buzzkill range at the graph. --Dgbrt (talk) 22:32, 20 August 2018 (UTC)
My point exactly - we now know quite a bit about the mass needed and process required to form a neutron star, making it unlikely the same mass would be able to form a black hole. I think that's what Randall meant in that part of the chart, but that's not what the explanation states. (Unfortunately, I've reached the point where I no longer want to argue with other editors over correct explanations.) Ianrbibtitlht (talk) 23:55, 20 August 2018 (UTC)
It seems intuitively possible, though. Imagine a black hole with the very lowest mass current theories predict they could form at, at the earliest point in time such a hole would be able to form. How much mass would it have shed through Hawking radiation since then? How far down into the neutron star weight class would it have gone by now? 11:33, 24 August 2018 (UTC)
Short answer: You probably couldn't measure it. Long answer: If black holes evaporate under Hawking radiation, a solar mass black hole will evaporate over 1064 years. This is a number far beyond any imagination. Our universe is 13.8 × 109 years old, or roughly 1010 meaning it would take the time of 1054 universes. 1054 equals to billion × billion × billion × billion × billion × billion (six times). And the smallest stellar black holes are not less than 2.4 solar masses. --Dgbrt (talk) 12:58, 24 August 2018 (UTC)
The Mysterious Eight Ball

How many of you remember the 8 Ball as a funny toy that you would ask questions and then turn over to receive an answer. Could that be the joke referred to in the 8 ball as a possible source of mysterious dark matter? --ProfKrueger (talk) 00:41, 21 August 2018 (UTC)

Explain xkcd: It's 'cause you're being physics-nerd-sniped! Ianrbibtitlht (talk) 00:09, 21 August 2018 (UTC)

Sorry to be picky, but I'm having trouble with "a star which was nearly in line with the sun appeared closer to the sun than usual." Doesn't a distant star's apparent position move away from the sun compared to the direct path? The light ray we see has been bent toward us, so it appears further away than an unaffected ray would, no? 03:31, 21 August 2018 (UTC)

You're not picky - you are just right. It's fixed. --Dgbrt (talk) 14:06, 21 August 2018 (UTC)

Can anyone explain how the paragraph associated with Buzzkill Astronomers has anything at all to do with a group of negative or skeptical astronomers? Am I misunderstanding the meaning of that phrase? If I'm just in the dark about some inside joke in astronomy, perhaps the explanation could enlighten me (and maybe others). As it reads right now, I don't see how anyone would find that explanation helpful. Ianrbibtitlht (talk) 03:31, 22 August 2018 (UTC)

My interpretation: "Black holes above a certain size would be impossible to miss [by astronomers]". In other words, the observations of astronomers rule out any dark matter candidates in that mass range. What a buzzkill, those astronomers, making those observations... Ahiijny (talk) 19:47, 22 August 2018 (UTC)
Monolith reference

(Spoiler alert for the movie "2001: A Space Odyssey") The monoliths in the movie were not just the three individual monoliths mentioned here. Near the end of the movie, a huge number of them appeared around, and apparently merged into, Jupiter. The added mass of the swarm of monoliths is what allowed Jupiter to initiate fusion, transforming it into the star Lucifer. So, the idea of "monoliths" being a source for dark matter is a joke on the final component of the plot of 2001, not just a vague reference. Wikipedia entry on Monolith (Space Odyssey) DanShock (talk) 19:05, 23 August 2018 (UTC)

Your spoiler applies to the sequel 2010: Odyssey Two and it's highly unrealistic. The mass of Jupiter is about 75 times smaller than the smallest possible star having fusion. Meaning the swarm of monoliths would have the mass of some 75 Jupiters. But Randall puts monoliths into the range of obelisks and pyramids less than 1019 kg. And if Jupiter would collect so much dark matter nothing would happen because dark matter doesn't react with normal matter except of gravitation. --Dgbrt (talk) 19:38, 23 August 2018 (UTC)
What about WIMPs and MACHOs?

https://en.wikipedia.org/wiki/Weakly_interacting_massive_particles https://en.wikipedia.org/wiki/Massive_compact_halo_object (talk) (please sign your comments with ~~~~)

Not covered by the comic but because both are well known I've entered both ruled-out theories into the explanation. --Dgbrt (talk) 12:21, 29 August 2018 (UTC)