The Milky Way Has A "Dark Twin"

Maybe Star Wars' dichotomous philosophy isn't so far off, because the galaxy we call home has a decidedly less starry counterpart. Astronomers have just discovered the Milky Way's "dark twin" lurking in the Coma cluster 300 million light years away. Named after the ultra-powerful telescope that initially found it, Dragonfly 44 is roughly the same mass (approximately one trillion solar masses, if galactic factoids are your thing) and structure as the Milky Way. But there's one crucial difference — Dragonfly 44 is made up of 99.99 percent dark matter.

According to the Washington Post, researchers first noticed the galaxy in 2015 as they were capturing the outskirts of galaxies using the Dragonfly Telephoto Array. When they detected what turned out to be Dragonfly 44, they used the Keck Observatory in Hawaii to continue their investigation. The galaxy is fairly close in the grand scheme of things, but its existence was overlooked thanks to its dim nature. Unlike the Milky Way, which houses over 200 billion stars, Dragonfly 44 is just 0.1 percent stars; without billions of stars shining out into the void, the galaxy is easy to miss through a telescope. In fact, according to Business Insider, there are so few stars that without another force at work, the galaxy wouldn't exist at all. However, something is holding those stars together, and that something is one of the most mysterious concepts in the universe.

So far, dark matter has largely remained an enigma. Scientists know it exists, but there's no way to detect it yet. According to the European Organization for Nuclear Research (CERN), the stuff we can see is just a tiny fraction of the universe — dark matter appears to make up the majority of the cosmos. I say "appear" because it's invisible to the instruments astronomers use today; dark matter emits neither light nor energy.

How do researchers know it exists if it's undetectable? Dragonfly 44 isn't the only galaxy that behaves in ways that don't fit our current understanding of physics. Some move faster than expected, and others, like Dragonfly 44, manage to stick together despite theoretically being too diffuse to exist. Scientists believe that dark matter is the invisible mass at work in these situations. Although they don't know what it's made of, they're able to approximate its location based on how light is distorted by different galaxies, and its gravitational effects can be seen at work throughout the universe.

This brings us back to Dragonfly 44. As lead author Pieter van Dokkum explained to Business Insider, researchers know the galaxy is composed of dark matter because if it was simply composed of a few stars, "all the other galaxies that are around it would pull and push on it and it would be deformed and ultimately completely destroyed." The galaxy's existence could prove to be instrumental in studying dark matter itself. Although other galaxies composed of dark matter have been found, Dragonfly 44 is the largest by far, giving researchers plenty to work with.

I'll leave you with a parting thought: I'm not saying there's a galaxy full of evil twins next door, but if evil twins were to colonize anywhere, we all know Dragonfly 44 would be their first choice.

Image: Giphy