Under the right conditions, however, dark matter can be coaxed to interact with either itself or with normal matter, dependent on its nature. If dark matter is made up of WIMPs (weakly interacting massive particles), then it has an annihilation cross section with itself and a scattering cross section with protons and neutrons that may render it sensitive to detectors. If instead, it’s made up of very light, low-mass particles known as axions, it may couple to photons under the right conditions. One of the experiments designed to search for axions is known as ADMX: the axion dark matter experiment. In 1983, physicist Pierre Sikivie invented the axion haloscope, which takes advantage of the fact that the axion-photon coupling can be amplified, with the right parameters, inside an electromagnetic cavity. Twelve years later, ADMX grew out of that research, and scientists have been searching for the axion ever since using that method.

To date, unfortunately, searches have come up empty. It’s possible that axions do not exist or, if they do, are not the dark matter, but it’s also possible that they simply exist with different parameters than what ADMX is sensitive to. It’s possible that a different electromagnetic cavity, with different properties, would enable interactions with axions. It’s possible that photon-axion interactions could take place, and that a cavity with the right properties could cause photons to scatter off of axions in a preferred direction. It might not seem likely, but it is conceivable that the EMdrive is such a cavity.