Using the world's most powerful telescopes, a team of astronomers has discovered what can be considered as a dark twin of the Milky Way. The galaxy that scientists called Dragonfly 44 is a little smaller compared with the Milky Way but it has about the same mass. What makes this Milky Way twin different from our own galaxy is that it consists mostly of the mysterious dark matter.
Using the W.M. Keck Observatory and the Gemini North telescope in Hawaii, astronomers discovered that the stars in Dragonfly 44 move through the galaxy faster than expected. The velocity of the stars indicates the mass of the galaxy. Faster star movement means the galaxy has more mass.
Researchers found that while the galaxy is as massive as the Milky Way, only one-hundredth of 1 percent of it is in the form of stars and what is considered as normal matter. The findings indicate that the galaxy is 99.99 percent dark matter. Dark matter is a hypothesized material that emits no light and has not yet been seen but may compose more than 90 percent of the universe.
Dragonfly 44 lies in the nearby Coma constellation. It is relatively close but because it is so faint, scientists did not realize until recently that it was a galaxy. It hardly has any star. The researchers said that if it were not for the presence of the dark matter, the galaxy would not even exist. It is so diffused that if there is nothing that holds it together, the Dragonfly 44 could blend with its environment.
"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," said study researcher Pieter van Dokkum, from Yale University.
Dragon 44 is not the first galaxy discovered that is primarily composed of dark matter. Ultra-faint dwarf galaxies also have the same composition. What makes the discovery different is that those galaxies were about 10,000 times less massive than Dragonfly 44. The newly found galaxy is an average-size galaxy, the kind that astronomers are more familiar with.
"If it's a very big or very large galaxy, you can brush it off and say, oh, that must be a rare thing but most of the stars in the universe live in galaxies this size," said van Dokkum.