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# 3 The nature of dark matter

Key evidence for the existence of dark matter includes: the rotation curves of spiral galaxies (i.e. the speeds with which stars move as a function of their distance from the centre of a galaxy); the galaxy velocities and gas properties of galaxy clusters; gravitational lensing by galaxy clusters; and the distribution of the intensity of the CMB radiation at different angular scales across the sky.

The cold dark matter (CDM) model of structure formation assumes that dark matter is some form of comparatively massive (and hence slow-moving) particle. An in-depth exploration of the physics of candidate dark matter particles requires knowledge of advanced particle physics and quantum field theory, which is not covered in this course. The following exercise explores some of the many candidates for a dark matter particle, and discusses some observational and experimental prospects for detecting them.

## Exercise 2

Read Section 2 of the article ‘Dark matter, dark energy and alternate models: a review’ by Arun et al. (2017), and answer the following questions.

1. What do the authors suggest are the most likely candidate dark matter particles? What reasons do they give for this?
2. How do direct detection experiments work, and why are they difficult to undertake?
3. What methods might enable the detection of axions?

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### Discussion

1. The two currently favoured dark matter candidates are weakly interacting massive particles (WIMPs) and axions. WIMPs could result from theories of supersymmetry that extend the standard model of particle physics. Axions are a popular candidate because – in addition to having the potential to act as dark matter – their existence would solve a problem in particle physics in which a symmetry of the strong force known as charge-parity symmetry is observed. Finally, primordial black holes are possible, but this theory may require more fine-tuning.

   Other candidates remain possible, but, as explained at the start of Section 2.4.4 of the paper, the three explanations outlined above are the mainstream options (i.e. they are considered the most likely).

2. Direct detection experiments for WIMPs look for very rare interactions of these particles with a large volume medium, such as liquid xenon or argon. Such experiments are difficult to undertake because, by their nature, dark matter particles are only expected to interact extremely rarely with ordinary matter, hence the need for very large volumes of target material. They must also be shielded from other particles that could produce spurious signals, so are typically located deep underground.

3. It is postulated that axions are converted to and from photons in the presence of strong magnetic fields. Laboratory and telescope experiments involving strong magnets aim to induce this conversion process and detect the resulting photons.

A particle model for dark matter is the current scientific consensus. However, it is important not to dismiss the possibility of alternative theories.

As with particle models, a full exploration of modified gravity theories requires physics beyond the scope of this course. However, the next exercise asks you to read two popular-science level discussions of such theories.

## Exercise 3

Read the two short articles below, which present contrasting views of modified gravity theories.

- ‘The case against dark matter’ (Wolchover, 2016)
- ‘Why modifying gravity doesn’t add up’ (Siegel, 2022)

Do you think Modified Newtonian Dynamics (MOND) is a better explanation for galactic orbits than dark matter? Which theory involves the least amount of ‘new physics’ that we don’t yet understand?

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OpenLearn - Unsolved problems in cosmology
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