Despite knowing dark matter is out there, astrophysicists are still missing one critical detail. “We know where it is and how much of it there is, but we don’t know what it is,” says Ben Safdi, an astrophysicist at the University of Michigan.

Some think this mysterious substance is made of hypothetical subatomic particles called sterile neutrinos. Unlike regular neutrinos, which interact via both gravity and the weak nuclear force, sterile neutrinos would only interact via gravity. And that seems to be the exact same rules of engagement that dark matter follows.

For this reason, some astrophysicists have spent the past few years searching for X-ray signals that would be produced if sterile neutrinos were decaying into normal matter as hypothesized. If such X-ray signatures were found, they would suggest sterile neutrinos are a (if not the) elusive source of dark matter. And since 2014, an unexplained X-ray emission line uncovered in a number of other galaxies has kept some astronomers hopeful we might be close to finally finding a fitting candidate for dark matter.

However, after sifting through 20 years’ worth of data from the darkest, blackest regions of our own Milky Way in search of that special X-ray signal, Safdi and his research team failed to find it. That means there is currently no experimental evidence that dark matter is made of the sterile neutrino, Safdi and his team concluded in their work, published in Science. After all, if the extragalactic X-ray signal were produced by dark matter (specifically the decay of the sterile neutrino), and the Milky Way is full of dark matter, then we also should spot the signal in our own galaxy, too.