Editing 2035: Dark Matter Candidates

{{w|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 of what dark matter could possibly be, charted by mass from smallest (given in {{w|Electronvolt#Mass|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, 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. [[Randall]] filled the gap between real small candidate particles and real large candidate objects with highly absurd suggestions.

An {{w|Axion|Axion}} is a hypothetical elementary particle that might be a component of dark matter.

{{w|Sterile neutrino|Sterile neutrinos}} are hypothetical particles interacting only via gravity. It's an actual candidate for dark matter.

{{w|Electron|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.) The mass of an electron is about 0.5 MeV which fits well into the graph.

A {{w|Neutralino|Neutralino}} is a hypothetical particle from {{w|Supersymmetry|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 {{w|Q-ball|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.

{{w|Pollen|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.  

{{w|Ceratopogonidae|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.

Insects of the clade {{w|Bee|Antophila}} a major pollinator of flowering plants.

In pool, the {{w|Pool (cue sports)|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.

Cows are Bovines extensively farmed on Earth for milk and meat. Although there is folk lore concerning cows {{w|Hey diddle diddle|acheiving  circum-lunar orbits}}, they have yet to be found elsewhere in the Universe.

;Obelisks, Monoliths, Pyramids

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 TMA-2 having a mass of about 500,000 tonnes).

;Black Holes ruled out by:

{{w|Black hole|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 {{w|Hawking radiation}}, and we'd see a buzz of gamma rays from every direction.

* GRB lensing: {{w|Gamma-ray burst|Gamma-ray bursts}} (GRBs) are the brightest events in the universe only been observed 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 often only detected by gamma-ray satellites in space but rarely at any other wavelengths. Measuring lensing effects would be very difficult. This [https://arxiv.org/abs/1406.3102 paper] discusses the probability of detecting lensing effects caused by Halo objects in the databases of GRB given sufficient objects to represent the missing mass.  

* Neutron star data: {{w|Neutron star|Neutron stars}} aren't black holes but they're also small high compact objects at about 1.4 and 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 stellar black holes which is far too low.

* Micro lensing: {{w|Gravitational microlensing}} is a gravitational lens effect. This is a prediction by Einstein's General Theory of Relativity and was first confirmed in 1919 during a solar eclipse when a star nearby the sun was closer to the sun than it should be. Astronomers have found many so called {{w|Einstein ring|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 the dark matter.

* Solar system stability: Our {{w|Solar system|solar system}} is 4.5 billion years old and very stable since then. If not we wouldn't exist. If dark objects at 10²⁴ kg - 10³⁰ kg (mass of Earth until Sun) would resemble the dark matter there should be many of them even 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

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). [https://www.youtube.com/watch?v=0fKBhvDjuy0 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.

;Title text

The title text refers to the fact that space is just vast emptiness where a little bit dust could be overseen. Actually the mean density of detectable matter in the universe is according to NASA equivalent to roughly [https://map.gsfc.nasa.gov/universe/uni_matter.html 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 {{w|Avogadro constant|6.022 x 10²³ atoms}}. Like at home using 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 unless the temperature goes to zero Kelvin which is impossible.