Dark matter is a theoretical type of matter that makes up the portion of the universe's mass that can't be observed. Although scientists don't yet know exactly what this mysterious substance is, they have nonetheless spotted a galaxy that might be composed almost entirely of it, reports Phys.org.

The strange galaxy, named Dragonfly 44, looks tiny upon first glance. It appears to be composed of an extremely small cluster of stars; a cluster so insignificant that there shouldn't even be enough gravity to hold them all together as a galaxy at all.

"Very soon after its discovery, we realized this galaxy had to be more than meets the eye. It has so few stars that it would quickly be ripped apart unless something was holding it together," said Yale University astronomer Pieter van Dokkum, lead author of the study.

A closer examination of the star cluster by Hawaii's W.M. Keck Observatory and the Gemini North telescope revealed that the stars were, in fact, being improbably held together, implying the existence of some other invisible source of mass. Further calculations suggest that Dragonfly 44 must have a mass similar to that of our own galaxy, the Milky Way, despite containing a fraction of the stars.

To put this into deeper perspective, only one-hundredth of 1 percent of Dragonfly 44's mass is in the form of stars or "normal" matter. The other 99.99 percent is dark matter. It's a massive dark galaxy, which would have been impossible to spot if not for the few stars hovering around in it.

"We have no idea how galaxies like Dragonfly 44 could have formed," said Roberto Abraham of the University of Toronto, co-author on the study. "The Gemini data show that a relatively large fraction of the stars is in the form of very compact clusters, and that is probably an important clue. But at the moment we're just guessing."

The discovery is mysterious largely because scientists don't even know what dark matter is. By studying galaxies like Dragonfly 44, however, maybe more can be learned about it. One thing seems certain: something is out there. Though we're looking right at it, it can't be seen. But it's there — whole galaxies of it, in fact.

Bryan Nelson ( @@brynelson ) writes about everything from environmental problems here on Earth to big questions in space.

Scientists discover a galaxy made almost entirely of dark matter
The dark galaxy has a similar mass to the Milky Way, but contains hardly any stars.