Our knowledge of what the universe is made of is woefully incomplete. The planets, stars and galaxies in the observable universe constitue just 5 percent of it, while the remaining 95 percent is “dark” — made of dark matter and dark energy.

What is this dark universe made of? Are dark matter and dark energy composed of just one kind of particle, or is there a whole raft of exotic particles out there, just waiting to be discovered?

These are perhaps the biggest questions today’s particle physicists are faced with.

One hypothetical class of particles that physicists believe dark matter may be made of is the “sterile” neutrino.

Neutrinos — once described as “the most tiny quantity of reality ever imagined by a human being” — are perhaps the most exotic and least understood of all known subatomic particles. Produced by the decay of radioactive elements, these particles rarely, if ever, interact with matter, making them extremely hard to detect and study. Every second, trillions upon trillions of neutrinos traveling at nearly the speed of light pass through Earth.

Currently, neutrinos are known to have three different types, or “flavors” — the electron neutrino, the muon neutrino and the tau neutrino. Each of these flavors can change into the other, “oscillating” spontaneously as they travel over long distances. These particles, on the rare occasions they interact with surrounding matter, do so via weak nuclear force. On the other hand, scientists believe that sterile neutrinos — if they exist — only do so through gravity.

Unfortunately, the hunt for sterile neutrinos has not been very fruitful. In August, researchers at the IceCube Neutrino Observatory in Antarctica revealed that they had failed to detect the particles, but had placed “strong limits on its possible existence.”

And on Friday, researchers associated with the Daya Bay Neutrino Experiment in China and the Main Injector Neutrino Oscillation Search (MINOS) collaboration at the Fermi National Accelerator Laboratory in the U.S. also reported that their hunt for the elusive particles had come up empty, even as they narrowed down the “phase space” where sterile neutrinos may be hiding.

“In the Big Bang at the very beginning of the universe, some of the energy turned into particles that so far have proven to be completely invisible,” Milind Diwan, a physicist at the U.S. Department of Energy’s Brookhaven National Laboratory, said in a statement. “That there exists such matter that is dark—undetectable—is an established fact. What that is composed of is a big mystery. Every time we find a hint of a new neutral particle that could account for this missing energy—or a new search method—it offers us a window into the universe that we must explore.”

The experiments were designed to observed neutrino oscillations. Scientists hoped that doing so would provide them a glimpse of a two-step flavor change — muon neutrinos briefly transforming into sterile neutrinos, which would make their presence known by the lack of any detectable signal, before morphing into electron neutrinos.

While the MINOS experiment looked at the rate at which the muon neutrinos disappeared, the Daya Bay experiment used a beam of electron antineutrinos and measured the rate at which they vanished.

Their conclusions were similar — if sterile neutrinos exist, there is now a very small “parameter region” where they may still be “hiding.”

“We can’t say that these light sterile neutrinos don’t exist, but the space where we might find them oscillating into the neutrinos we know is getting narrower,” Alex Sousa from the University of Cincinnati, one of the scientists associated with MINOS, told Symmetry magazine.