Neutrinos are tiny ubiquitous particles that are generated by the fusion of stars and other highly-energetic interactions. They are nearly massless and travel close to the speed of light. We are constantly irradiated by them, but are unaffected because they rarely interact with more familiar forms of matter.
The standard model of physics allows for three neutrino flavors, but there have been a number of anomalies in experimental results that suggests this is not the whole picture. Some physicists (like the authors of this article) suggest these anomalies point to a fourth neutrino flavor.
This fourth neutrino flavor, the sterile neutrino, would interact with the universe only through gravity (and not the strong nuclear, weak nuclear, or electromagnetic forces like other particles).
Some fun facts:
The “dark sector” (including dark energy, dark matter, and the sterile neutron if it exists) accounts for 95% of the universe’s energy density.
Neutrinos oscillate through the various flavors (electron, muon, tau) as they travel (this discovery earned the 2015 Nobel Prize in Physics).
Most of the experimental work seems to be attempts to measure the oscillations of neutrinos over varying distances from a neutrino source (particle accelerator, nuclear fission reactor) and then figuring out how to account for the observed results.
Particle-wave duality: all particles act as both waves and marbles. The wavelength of high-energy particles is shorter than low energy particles and this affects how it interacts with matter.
Experimental design is crazy for this kind of work: vats with many tons of liquid argon, particle accelerators, etc.