Deuterons — atomic nuclei made up of a proton and a neutron — are thought to form in the same way as helium-3 nuclei, in collisions between primary helium-4 nuclei and other nuclei in the interstellar medium. If that is indeed the case, the deuteron-to-helium-4 flux ratio should be similar to the helium-3-to-helium-4 flux ratio. But this is not what the Alpha Magnetic Spectrometer (AMS) on board the International Space Station sees. Aguilar...
