It’s a real-life space oddity.

Astronomers have discovered an ancient galaxy that seems to hold little if any dark matter — an invisible substance that astronomers previously thought was an essential ingredient for galaxy formation.

The 10-billion-year-old galaxy, known as NGC 1052-DF2, is located 65 million light-years away in the constellation Cetus. It's as large as our Milky Way galaxy but contains 400 times less dark matter than the astronomers had expected to find. The surprising discovery, described in a paper published online March 28 in the journal Nature, raises new questions about how galaxies form.

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“You don’t expect a galaxy to have no dark matter because dark matter is not something a galaxy can just opt out of,” said Dr. Pieter van Dokkum, a professor of astronomy and physics at Yale University and the leader of the team of astronomers who made the discovery using a pair of ground-based telescopes and the Hubble Space Telescope.

The newfound galaxy also seems to lack a black hole at its center. That came as another surprise, since astronomers have long thought that most galaxies contain black holes.

Dark matter is thought to make up about 27 percent of the universe, but because it emits no light it cannot be observed directly. Instead, scientists infer its existence based on its gravitational effects on galaxies and galaxy clusters.

It's long been believed that galaxies arise when dark matter attracts gas that ultimately coalesces into stars — and that once a galaxy forms, dark matter helps hold it together.

But Dr. Jeremiah Ostriker, a Columbia University astrophysicist who was not involved with the study, said this atypical galaxy might have been formed by the collision between two typical galaxies, leaving a region of space without dark matter but with gas that eventually turned into stars.

Another possibility, Ostriker said, is that NGC 1052-DF2 contains a form of dark matter that is less massive than the "cold" dark matter thought to make up typical galaxies.

Whatever the explanation, Ostriker said the galaxy's discovery amounted to a "death knell" for the notion that dark matter might not exist at all.

The next challenge for researchers will be looking for more examples of galaxies without dark matter. “It would be extremely exciting if there were more, or even if this was a common type of galaxy,” van Dokkum said. “So the hunt is on.”

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