Dark matter is made of axions

The argument

Scientists postulated axions to account for the neutral charge and symmetry of neutrons in the nucleus, and the argument begins with that original problem before turning to dark matter. Neutrons are made of quarks which are smaller particles that have charge. Neutrons overall are neutral and have symmetrical distribution of charge. That combination is not automatic: an object built from charged components could in principle carry an uneven internal arrangement, so the observed symmetry calls for an explanation rather than being a default. Scientists predicted that there must be an energy forcing the charges to distribute equally and leading to zero net charge on the neutrons, and particles like axions may account for the energy and solve this problem. The second move is that a particle proposed for one purpose turns out to satisfy the requirements of another. Axions also fit the two criteria necessary to make up dark matter. First, there is a massive number of axions which allows scientists to postulate that they can make the large missing energy in the universe known as dark matter — quantity matters, because whatever dark matter is, there must be enough of it to account for the shortfall. Second, they are collision-less which is consistent with the properties of dark matter, since dark matter is inferred precisely from its failure to interact in the ways ordinary matter does. The axion theory attracts scientists because it can solve two enigmas at once, making it more likely to be true. A hypothesis introduced to answer one question, and then found to answer an unrelated one without adjustment, is doing more work than a proposal tailored to each in turn. The axion theory explains why the universe is heavier than it looks and it explains the neutral charge of neutrons. Many scientists consider axions as good candidates for dark matter.

Premises

Counter-arguments

It is difficult to conclude that axions are what make up dark matter. Scientists have yet to detect axions. Axions are so light that they cannot be seen or be received on a detector. Also, they do not emit light that scientists can measure. They do not collide with normal matter but pass through it. Collision releases energy and quantifying the energy would indicate the presence of particles. Because of all of these obstacles, axions are still hypothetical particles.

Rejecting the premises

[Rejecting P1] A motivation is not evidence, and the description does not match the puzzle it invokes: what needs explaining is why the neutron's electric dipole moment is so nearly zero when the theory permits it to be large, not that quarks carry charge. A particle postulated to solve a problem is not thereby shown to exist. [Rejecting P2] Meeting two criteria makes something a candidate, while the position asserts a composition. Sterile neutrinos, WIMPs and primordial black holes also satisfy those criteria, so the criteria do not select this answer, and solving two puzzles at once is an appeal to elegance rather than to evidence. The counter names the decisive gap: no axion has been detected.