Astronomers have discovered a galaxy as big as the Milky Way that consists almost entirely of dark matter, a mysterious and invisible substance that scientists have been trying to figure out for decades, reports Space.com. Only one-hundredth of one percent of the galaxy is ordinary, visible matter like stars and planets. The other 99.99 percent of the stuff in this galaxy can't be seen.

No one really knows what dark matter is made of, but scientists believe it exists because they can see the effects of its gravity on other things in space. Whatever it may be, about 80 percent of the mass in the universe is dark matter.  

This dark galaxy, named Dragonfly 44, was first detected in 2015, through the use of the Dragonfly Telephoto Array in New Mexico. With a combination of eight telephoto lenses and cameras, the array is designed to look at objects in space that aren't bright enough to see with other telescopes. 

Dragonfly 44 is one of 47 ultradiffuse, or "fluffy" galaxies that Pieter van Dokkum of Yale University and colleagues found in the Coma Cluster, a group of at least 1,000 galaxies around 300 million light-years from Earth. This distance is easily close enough for a telescope to see; the Hubble Space Telescope can see billions of light-years away. But no one had noticed these galaxies hiding in the dark before. Dragonfly 44 was one of the largest and brightest galaxies they found. While it's as big as the Milky Way, it only emits about 1 percent as much light.

Van Dokkum and his team later realized that there was something very odd about Dragonfly 44: a galaxy that big couldn't possibly hold itself together with so few stars. There wouldn't be enough gravity, and the stars would drift apart. They suspected that dark matter was responsible for holding the galaxy together, and this particular galaxy seemed like it contained a ton of it, so they set out to determine exactly how much.

To investigate the amount of dark matter in Dragonfly 44, the team turned to one of the largest telescopes on Earth, located at the W. M. Keck Observatory in Mauna Kea, Hawaii. They used a tool on the Keck II telescope called the Deep Imaging Multi-Object Spectrograph (DEIMOS) to study the movement of stars in the galaxy.