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Astronomers Discover Milky Way–Sized Galaxy That's 99.99% Dark Matter

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At left, a wide view of the Dragonfly 44 galaxy, and at right, a close-up of the same image, revealing its large, elongated shape and halo of spherical clusters of stars around its core, similar to the halo that surrounds the Milky Way. Image credit: Pieter van Dokkum, Roberto Abraham, Gemini Observatory/AURA

 
An unusual galaxy, made up almost entirely of exotic “dark matter," has left astronomers and physicists scratching their heads. The galaxy, known as Dragonfly 44, is located some 300 million light-years from our own Milky Way and is about the same size as our galaxy—but a mere 100th of 1 percent of it is made up of ordinary matter. The rest—99.99 percent—is dark matter.

Dragonfly 44 actually has about as much dark matter as our galaxy, but it has far fewer stars. As a result, the dark matter almost completely dominates. “It’s kind of a dark twin of the Milky Way,” lead researcher Pieter van Dokkum of Yale University tells mental_floss.

The findings were published today in Astrophysical Journal Letters [PDF].

First proposed in the 1930s, dark matter is a mysterious form of matter believed to account for more than one-quarter of the mass and energy in the universe. (A larger proportion—more than two-thirds—is the even-more-mysterious dark energy; a mere 5 percent of the universe is made of ordinary, visible matter.) Dark matter doesn’t interact with ordinary matter—it can’t be seen with optical or radio telescopes—but its presence can be deduced through the gravitational tug that it exerts.

The fact that dark matter dominates over regular matter is not by itself a surprise: In most galaxies, van Dokkum explains, there’s about 50 times as much dark matter as ordinary matter. But in Dragonfly 44, that ratio is even more extreme, thanks to the lack of stars.

The only other galaxies known to be this heavily skewed toward dark matter are the small dwarf galaxies that orbit the Milky Way. But Dragonfly 44 isn’t like those galaxies—rather, it’s just as large and massive as the Milky Way itself. How it ended up so dark matter–heavy, and with so few stars, is a mystery. “We thought we understood these [more massive] galaxies quite well,” says van Dokkum. “They usually have a relatively small amount of dark matter, in proportion to the number of stars that they have. This galaxy turns that on its head.”

Because of the paucity of stars, Dragonfly 44 is extremely faint. It’s one member of a new class of diffuse, dim galaxies discovered recently using the Dragonfly telescope array, an innovative imaging system that uses ultra-“fast” commercial telephoto lenses (the kind that sports photographers use) to find dim objects in the night sky. The brainchild of van Dokkum and University of Toronto astronomer Roberto Abraham, Dragonfly was tailor-made to detect objects with “low surface brightness”: While the light from stars is concentrated in specific points in the sky, galaxies are dim and their light is spread out—and these peculiar galaxies are even dimmer, and thus even harder to see. “These objects had always been missed, but with the Dragonfy telescope, we found them,” van Dokkum says.

Later, he and his colleagues aimed Hawaii’s Keck telescope at Dragonfly 44 for a closer look (because the galaxy is so dim, this required collecting data over six nights). They were able to measure the speeds of some of the galaxy’s stars, from which the total mass of the galaxy can be calculated. From the brightness and the mass, they determined how much mass is “missing”—that is, they inferred how much extra mass must be present in the form of dark matter, so as to keep the galaxy from flying apart. Observations with the Gemini North telescope, also in Hawaii, revealed a halo of spherical clusters of stars surrounding the galaxy’s core—similar to the halo known to surround our own Milky Way. “Ultimately, we may learn about the connection between dark matter and these mysterious star clusters,” van Dokkum says.

Meanwhile, the biggest mystery of all remains the identity of the dark matter itself. Physicists’ best guess is that it’s made up of some kind of primordial particle, perhaps created at the time of the big bang—but numerous attempts to detect such particles directly (including the most recent effort) have come up empty. And Dragonfly 44, being so far away, isn’t likely to help much—but in principle, other dark matter–dominated galaxies could still be awaiting detection, much closer to home. “If we found a galaxy like this, that’s close to us—that might be the ideal place to look, to make a direct detection of the dark matter particle,” van Dokkum says.

David Spergel, a Princeton University astrophysicist who was not involved in the current research, tells mental_floss that these low surface–brightness galaxies “are useful ‘laboratories’ both for studying the properties of dark matter and understanding galaxy formation.”

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Space
Is Your Town in the Path of the Next Total Solar Eclipse?
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On August 21, people across the continental U.S. got to witness a spectacular total solar eclipse. From Oregon to South Carolina, those in the path of totality saw the world go dark as the Moon completely covered the Sun. Even people hundreds of miles away from the path were able to see a partial eclipse.

The highly anticipated event was rare because of the path it carved across the U.S. Total solar eclipses happen often enough, but 2017 was the first year one has been visible from the contiguous U.S. since 1979.

If you didn’t get to see the full eclipse this year, you’ll get a second chance—as long as you’re willing to wait a few years. The next total eclipse to cross the continental U.S. will occur in 2024, traversing parts of Mexico before heading up through Texas, across the Midwest, and past Buffalo and northwestern New England before crossing the Atlantic Ocean.

Future eclipses
Lucy Quintanilla

On April 8, 2024, cities like Dallas and Killeen, Texas; Hot Springs, Arkansas; Poplar Bluff, Missouri; Bloomington, Indiana; Cleveland, and Buffalo will get to witness the beauty of the solar corona, an event that dedicated eclipse chasers—who travel the world in pursuit of eclipse experiences—alternately describe as terrifying, weird, thrilling, mind-blowing, life-changing, and not entirely different from taking psychedelics.

See if you'll be in the path of totality come 2024 here. If you're going to travel to see it, you’d be smart to book your accommodations well in advance, since lodging in cities in the path of totality can book up years before the event. And this time, don't forget the eclipse glasses.

If you won’t be in the Midwest or New England in 2024, you’ll have another chance at an American eclipse viewing eventually. In August 2045, a total solar eclipse will journey across northern California through the continental U.S. and down to Florida and the Caribbean, passing over major cities from Reno to Miami along the way.

If you don’t want to wait until then, take a look at this infographic to see where across the world you can find an eclipse sooner. But our recommendation? Book yourself a room in Hot Springs for six years from now and prepare to take a rejuvenating soak while you watch.

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Can’t See the Eclipse in Person? Watch NASA’s 360° Live Stream
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Ian Hitchcock/Getty Images

Depending on where you live, the historic eclipse on August 21 might not look all that impressive from your vantage point. You may be far away from the path of totality, or stuck with heartbreakingly cloudy weather. Maybe you forgot to get your eclipse glasses before they sold out, or can't get away from your desk in the middle of the day.

But fear not. NASA has you covered. The space agency is live streaming a spectacular 4K-resolution 360° live video of the celestial phenomenon on Facebook. The livestream started at 12 p.m. Eastern Time and includes commentary from NASA experts based in South Carolina. It will run until about 4:15 ET.

You can watch it below, on NASA's Facebook page, or on the Facebook video app.

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