Dark matter is a theoretical type of matter that has been posited to make up about 80 percent of all matter in the universe. Without it and its gravitational weight, galaxies like our own Milky Way wouldn't have enough gravity from normal matter to hold themselves together; galaxies everywhere would be flinging themselves apart.

While there's good reason to believe that dark matter is real (after all, galaxies aren't flinging themselves apart everywhere), we have no idea what it actually is. That's why it's called "dark": whatever it is, it's invisible and mysterious.

Three years ago, hope sprang eternal that the dark matter mystery might finally be solved after scientists discovered a bizarre galaxy about 1.3 billion light-years from Earth that appeared to have become separated from the dark matter surrounding it. Such a situation would fit with predictions that dark matter interacts with forces other the gravity, which would be a big boon in the quest to uncover what dark matter is. If it interacts with more than gravity, then there are more ways in which it can be examined.

But now, unfortunately, it appears that those hopes have been shattered. A more thorough investigation of that bizarre galaxy has shown that it hasn't separated from its dark matter after all, reports Phys.org. Its dark matter is exactly where it should be if dark matter only interacts with the force of gravity.

And so, dark matter is still very, very dark, and the mystery only deepens.

"The search for dark matter is frustrating, but that's science. When data improves, the conclusions can change," said lead author Dr. Richard Massey. "Meanwhile the hunt goes on for dark matter to reveal its nature."

He added: "So long as dark matter doesn't interact with the universe around it, we are having a hard time working out what it is."

The new results were found thanks to the Atacama Large Millimeter Array (ALMA) in Chile, South America, which provided a higher resolution than the previous data collected with the Hubble Space Telescope allowed. Even though the new results indicated that dark matter does not interact, they don't prove definitively that it doesn't. It's still possible that dark matter interacts, just at a very low — and currently undetectable — amount.

Researchers are holding out some hope that some other future discovery might flip the script again.

"We will keep looking for nature to have done the experiment we need, and for us to see it from the right angle," said Dr. Andrew Robertson, who recently presented the new results.