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There might be a galaxy made mostly of dark matter orbiting the Milky Way!
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Reid: We can only see about 4 percent of our universe. The rest is mysterious, invisible dark energy and dark matter, which scientists still don’t know very much about. Obviously, they’d like to learn more about the other 96% of the universe. But how do you study something you can’t see?

Well, one option is to study the dark parts of our universe indirectly, by looking for ways they might be influencing the visible universe. And earlier this year, an international group of astronomers confirmed that some of those clues pointed to a small, undiscovered galaxy, mostly made of dark matter, orbiting our own galaxy. It’s being called Galaxy X.

Back in the 1970s, astronomers were measuring the rotational speeds of galaxies, and they noticed something surprising: the objects as the edges of galaxies had velocities that didn’t fit with the astronomers’ calculations.

The researchers were predicting the movement of objects in the galaxy based on the amount of visible matter, and its gravitational influence. But these things were moving way faster than they should have been. They realized that for the velocities of these objects to make sense, there had to be gravity from invisible, dark matter affecting them too.

More recently, astronomers have found small, dim galaxies, like the Draco Dwarf Galaxy and Triangulum II, that appear to have much more gravitational force than they should, suggesting that they’re packed with dark matter. Physicists have even guessed that there might be galaxies mostly, or entirely, made up of dark matter: dark matter galaxies.

Back in 2009, two researchers -- Sukanya Chakrabarti and Leo Blitz -- analyzed variations in the distribution of gas at the edges of the Milky Way, and they found that these so-called “ripples” could be caused by a clump of dark matter orbiting our galaxy.

In other words, they’d found evidence of a new dwarf galaxy, about a hundredth the size of our own and made almost entirely of dark matter. If it’s really out there, Galaxy X would have skimmed the edge of our galaxy a few hundred million years ago, and would now be about 300,000 light-years from the Milky Way’s center -- about 250,000 light-years farther than the galactic rim.

But that’s the thing -- they didn’t know if it was out there. The math made sense, but they needed more evidence. Since they had a basic idea of where Galaxy X should be, they and a few other researchers looked for clues in that area, in the form of the small amount of visible matter that might live in the mostly-dark galaxy. And in 2015, they published a paper saying that they’d found 4 of a kind of star called a Cepheid variables in that area of the galaxy.

Cepheid variables are pulsating stars, meaning that they change in apparent brightness -- the amount of their light that reaches Earth -- in a predictable way. By measuring that change in brightness, astronomers can calculate how much light the star is actually putting out -- its luminosity. And from there, they can figure out the distance from Earth to the star.

All four of the stars were about 300,000 light-years from the center of the galaxy, and at roughly the same angle as the rest of the galactic disk -- just about where Galaxy X was predicted to be. Of course, these stars could still be part of our own galaxy. But that would be weird, because no other Cepheid variables have been found that far from the center of the galaxy, but still close to the angle of the galactic disk.

And the researchers found four clustered together! So it’s possible that these stars are some of the bits of visible matter in Galaxy X. The team’s next step was to figure out if the stars were moving with the same speed they predicted for Galaxy X -- around 200 kilometers per second. And in January of this year, they announced that they’d analyzed 3 of the Cepheid variables -- the ones they had data for -- and they are moving at about the same speed as Galaxy X should be.

We still don’t know for sure if these stars mark the location of Galaxy X. But if researchers find enough evidence to conclude that they do, then we’ll have found a dark matter galaxy by studying the ripples in the gas and dust at the edge of the Milky Way. And maybe, if astronomers spend more time analyzing these ripples, they’ll find more dark matter galaxies, and help unveil a tiny bit of the darkness that dominates our universe.

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