Based on how gravity affects the gases at the edge of our galaxy, we should have a satellite galaxy located about 26,000 light-years away...except nobody's ever seen it. This might be because it's composed almost entirely of dark matter.
Astronomer Sukanya Chakrabarti looked for gravitational effects created by potential satellite galaxies of our home, the Milky Way. Not unlike the gas giants such as Jupiter of Saturn, the Milky Way has a bunch of relatively tiny "moon" galaxies. The most famous of these, the Magellanic Clouds, are about 10 percent the size of our galaxy, but most of these satellite galaxies are less than a hundredth the size of the Milky Way.
Back in the 19th century, astronomers detected the planet Neptune by observing the slight gravitational wobble of Uranus, which could only be caused by an undetected planet further out. The same basic principle is at work here, as this mysterious Galaxy X is creating movement in the gas at the edge of our galaxy. Chakrabarti suspects the galaxy is composed mostly of dark matter, which is part of the reason why we hadn't found it up to this point.
If it exists, Galaxy X would be about 1% the size of the Milky Way, making it the third largest satellite after the Magellanic galaxies. While dark matter would make up most of its mass, there would likely still be some dim stars of regular matter in there as well. The galaxy lies on the same plane as our galactic disc, meaning astronomers need to look through all the bright lights of the Milky Way just to see Galaxy X. Still, now that we know where to look, Chakrabarti says, we should be able to find it.
There's some theoretical basis to think that Galaxy X really is out there. Our understanding of galactic formation tells us that our galaxy should have hundreds of satellite galaxies, but we've only found a few dozen. The idea that "dark" galaxies are hiding out there, unable to be seen in the visible light spectrum, could go a long way to sorting that mystery out. If we can find the dim star lights of Galaxy X, then we might be able to see the darker parts of the galaxy with infrared telescopes.
The search is now on for Galaxy X, and Chakrabarti says that, if we find this galaxy, there's every chance we'll detect the existence of fellow dark matter Galaxies Y and Z and so on. If we can't find this galaxy, then she argues that something must be wrong in the calculations. This might mean the halo of dark matter known to surround the Milky Way is different from what we thought it was, which would open up some new avenues of research. Either way, the hunt for Galaxy X figures to teach us some crucial new information about elusive dark matter.