After hydrogen and helium, lithium is the lightest and simplest element in the universe. It should have been everywhere right after the Big Bang...but the data shows a mysterious shortage. The explanation may point to an unlikely dark matter candidate.
The leading explanation for the creation of the lightest elements at the beginning of the universe is known as Big Bang Nucleosynthesis. As Physics World explains, the idea is that the primordial universe was once a thick soup of super-heated protons and neutrons. Those two particles began assembling into atomic nuclei, creating the hydrogen isotope deuterium as well as helium and lithium isotopes. Electrons entered the picture as temperatures dropped, and the afterglow of this process is what we now know as the Cosmic Microwave Background.
There's just one problem with this whole idea - our observations of the CMB suggest that there isn't enough lithium-7. In fact, there's only about a third the predicted amount, which is a fairly gaping hole in an otherwise satisfying hypothesis. To fill that gap, University of Florida physicists led by Pierre Sikivie point to the axion, a hypothetical particle that has long been discussed as a possible alternative candidate for dark matter. If the axion exists, it's a very light particle that barely interacts at all with matter. (For more on the axion, and other dark matter candidates, check out our primer.)