The Milky Way's 'dark twin' might help us solve one of the biggest mysteries in science

Flickr/Abdul Rahman

Somewhere in the universe, hundreds of millions of light-years away, the Milky Way has a twin.

This twin, called Dragonfly 44, is a bit smaller than the Milky Way — about 60,000 light-years across — and has pretty much the same mass.

But this twin has a dark secret.

Using the most powerful telescopes on the planet, a team of scientists found that almost the entire galaxy, about 99.99% of it, is made up of some of the most mysterious stuff in the universe: dark matter.

The findings were published in The Astrophysical Journal on Thursday.

The glue of the universe

So what is dark matter anyway? Only one of the biggest enigmas in science.

It all stems from this fact: Eighty-five percent of the mass of the universe is unaccounted for. Just let that sink in for a minute. That means all of the people and mountains and planets and stars that you see around you only make up 15% of the mass of the universe.

The rest of it is dark matter.

Flickr/Abdul RahmanDark matter is kind of the cosmic glue that holds the universe together.

Even though we can't see it, we know it exists because of how stars move through space.

The pull of gravity should account for the motion of the stars. But when scientists add up all of the ordinary matter in the universe — all the other stars and planets and cosmic junk — the stars actually move much faster than they should if they were being moved by gravity alone.

And it's not just stars. Scientists have seen this strange effect at work on entire galaxies.

And if all of the matter we see isn't enough to tug around stars and galaxies at these speeds, something else, something invisible, must be pulling on these objects. And that something is what scientists call dark matter.

But it's so feeble that even our most sensitive detectors haven't been able to pick up its signal since it was first dreamt up in 1922.

So the bottom line is we don't really know what dark matter is, making the vast majority of the universe a complete mystery.

But now that scientists have found this heavyweight galaxy made up almost entirely of dark matter, we might be on our way to finally putting this cosmic mystery to rest.

The Milky Way's long-lost twin

Although Dragonfly 44 is relatively close, it's so faint that until recently, scientists didn't even realize it was a galaxy. That's because the galaxy has hardly any stars. Take away 99 of every 100 stars in the Milky Way, and that's how many stars this galaxy is working with.

In fact, the galaxy has so few stars that if it weren't for the dark matter, it wouldn't even exist.

"If the galaxy only had those stars and no dark matter to hold it together, all the other galaxies that are around it would pull and push on it and it would be deformed and ultimately completely destroyed," Pieter van Dokkum, lead author of the study, told Business Insider. "It would be too feeble to hold together in that violent environment."

Galaxies like Dragonfly 44 are so diffuse that if nothing held them together, they would quickly blend with their environments.

You can think of it like dropping a cup of Coke in a swimming pool, van Dokkum said. The Coke would quickly disperse in the pool and disappear. But if you dropped something much heavier and more solid, like a bowling ball, it would stay together.

If dark matter is invisible, how do we know it's there?

Flickr/Abdul RahmanUsing the Keck and Gemini observatories in Hawaii, the scientists looked at the light of the stars to figure out just how fast they're moving through the galaxy. And it turns out they're a lot speedier than you might expect.

This is the key to figuring out the mass of a galaxy.

"It's not how bright the galaxy is, but the movement of the stars that tells you how much matter there is," van Dokkum said. "And then we say: This is the total mass of the galaxy, this is how much is contributed by the stars, and the rest must be dark matter. And it turned out to be a huge amount."

It's not weird for a galaxy to have dark matter. A good chunk of our galaxy is made up of these mysterious particles. But this galaxy, a trillion times the mass of the sun, is about as heavy as the Milky Way, and almost entirely dark matter. And right now, it's bringing up more questions than answers.

"There are several things that need to be understood about Dragonfly 44," van Dokkum said. "One is: How can a galaxy like that form? We always thought we understood the history of galaxies like the Milky Way — how they start out and grow and how stars form in them. But all of a sudden we find this object that looks like the Milky Way, but it has 100 times less stars. So just understanding how that could have happened is a complete mystery right now."

New hunting grounds

Dragonfly 44 is not the first galaxy scientists found to be made almost entirely of dark matter, but it is the most massive.

Until now, scientists were searching tiny dwarf galaxies around the Milky Way for dark matter. But because this galaxy is so much more massive, van Dokkum believes it might be a better candidate for actually detecting the very feeble dark-matter signal.

And now that we have this mysterious, physics-defying dark twin in our sights, we might be on the path to actually detecting dark-matter particles for the first time.

Flickr/Abdul Rahman

"There's lot of effort going into trying to understand what the dark matter is," van Dokkum said. "It's one of the biggest mysteries in science. It dominates the mass of the universe, and we have no idea what it is. The hope is that by looking at galaxies that are made up almost entirely of dark matter, where there are very few other stars to in the galaxy to confuse us, we can try to detect signals of the dark-matter particle."

What comes next

Although Dragonfly 44 is a great start, it's just a little bit out of our range if we want to detect our first dark-matter particle. So scientists are already looking for other dark-matter Milky Way twins in our own neighborhood.

Scientists are also looking for galaxies that might be even darker than this one to find out if there's a limit to how few stars a galaxy can have.

"Are there pure dark matter galaxies? That would be incredibly exciting. This galaxy is almost entirely dark matter, but we want to see if there are any pure dark-matter galaxies out there with no stars at all," van Dokkum said. "We'll be looking at fainter and fainter objects to try and see if there are even darker twins of the Milky Way. We want to see if the family extends to even darker and fainter objects."

Although van Dokkum said they have a ways to go before solving the mystery of dark matter, these galaxies might hold the key.

"I think the exciting thing is that there might be these incredibly dark-matter dominated, or even pure dark-matter objects that are very massive," van Dokkum said. "The hope is that by studying these galaxies we might learn what the dark matter is. Objects like Dragonfly 44 give us some hope that we can push this technique to a much greater sensitivity."

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