What if dark matter was more like a field than a particle...and how does that relate to string theory?
The standard picture of dark matter is one of individual particles: annihilating with each other, bumping into nuclei or electrons in direct detection experiments, or being produced from particle collisions. This is a good description if the dark matter mass is larger than a few electron-volts (eV). But if the dark matter is much lighter, and is a boson rather than a fermion, quantum mechanics tells us that the dark matter particles lose their individual identities and start behaving collectively, as a field. In fact, there is so much dark matter that the quantum nature of this field is not apparent, and it can behave like a classical field. Photons do exactly the same thing: the light from your computer screen that allows you to read this text is best described by an electromagnetic field which satisfies Maxwell's equations, rather than by individual photons. But just as you would never want to derive Ampere's law for currents by adding up Feynman diagrams, the intuition for experiments to look for coherent-field dark matter requires thinking about charges and currents, rather than individual scattering events.
The axion is a dark matter candidate which can satisfy all the required properties to behave as a coherent field. There are lots of reasons to expect ultralight axions, both experimental and theoretical. The observation that an electric dipole moment for the neutron has not yet been detected, despite the fact that it is easy to write down a term which would generate one, is known as the "strong CP problem." An ultralight axion could solve this problem by essentially cancelling off this extra term. Highbrow theoretical arguments from string theory also suggest the existence of axions, maybe even hundreds of them, which are responsible for stabilizing the extra dimensions which string theory requires for consistency. Generically, axions will interact very weakly with electromagnetism, which suggests a detection strategy: in a strong magnetic field, the axion field will source a very weak response magnetic field. Along with colleagues at MIT, I recently proposed a design for an experiment "ABRACADABRA" to detect axion dark matter, which exploits advances from totally different fields (MRI research in medical physics, and ultrasensitive current measurement in metrology) to search for axions over a wide range of masses.
Axion dark matter also leads to some pretty interesting astrophysical signals. Since the axion field can convert to electromagnetic radiation in the presence of strong magnetic fields, the best place to look for axion conversion is near the strongest magnetic fields in the universe: the region surrounding magnetars, neutron stars with absurdly large magnetic fields. So large, in fact, that everyday physics changes: hydrogen atoms are cylindrical rather than spherical, and photons of different polarizations travel at different speeds even in vacuum. My collaborators and I calculated the axion-photon conversion probability in the highly-magnetized plasma surrounding magnetars and other high-field neutron stars, and found that radio telescopes might be able to detect axion signals from a magnetar very close to the black hole at the center of the Milky Way.
A. Hook, Y. Kahn, B. Safdi, and Z. Sun. Radio Signals from Axion Dark Matter Conversion in Neutron Star Magnetospheres. arXiv:1804.03145.
SQUID Magnetometer Detection of Axion Dark Matter (video). KITP conference "Symmetry Tests in Nuclei and Atoms", September 22, 2016.