I want to know what dark matter and dark energy are comprised of. They remain a mystery, a complete mystery. No one is any closer to solving the problem than when these two things were discovered. -Neil deGrasse Tyson
Scientists recently discovered that a galaxy identified in 2015 is actually a new type of galaxy than we previously knew existed. The galaxy, called Dragonfly 44, is not like our galaxy - it’s a galaxy made up almost entirely of what’s called dark matter. But what is dark matter? How did astrophysicists learn this about Dragonfly 44? And why has it got them so excited?
Dark Matter
What is it?
The truth is, we don’t really know what dark matter is. In fact, there are still some scientists that don’t believe it exists.
...huh???
Ok, here’s the scoop. For years, scientists have been observing a few weird things in the Universe. Not just unusual - things they couldn’t explain. For instance, scientists use the movement of large objects in space to calculate their mass (the amount of stuff that makes up an object), and they were finding objects moving much faster than could be explained by the mass they could detect. And they were seeing light waves that were being affected by gravity, though there seemed to be no object around to create the gravitational force. These observations led scientists to one of two conclusions: either Einstein's theory of general relativity, which perfectly explains how gravity works in some ways, is in other ways wrong...or there exists a different kind of matter that we, so far, know nothing about. Most scientists believe in this different kind of matter, although we’ve never been able to observe it; it’s come to be known as dark matter.
What isn’t it?
While we might not know a whole lot about what dark matter is, we know a fair amount about what it is not:
- Dark matter does not absorb or emit light - in other words, dark matter is not visible.
- Dark matter is not made up of normal matter.. Normal matter is baryonic, or made up of baryons - the word for protons and electrons. If dark matter were made up of baryons, even in cloud formations, it would be detectable through reflected light.
- Dark matter is not antimatter, which collides with and annihilates regular matter to produce gamma rays (nowhere near enough gamma rays have been detected for dark matter and antimatter to be the same).
- Dark matter is not rare; in fact, it’s estimated to make up about 27% of the Universe, whereas regular matter (the stuff that makes up us, the Earth, stars, galaxies, and any other seeable objects) makes up only about 5%. (The rest of the Universe is most likely made up of dark energy, and, just like dark matter, we don’t really know what that is either, only that it’s responsible for the Universe expanding faster than before.)
Dragonfly 44
Dragonfly 44 is a galaxy that’s not as far away as others we’ve observed but went unnoticed due to its darkness. While the galaxy is about as large as our own, the Milky Way, it emits only about 1% of the light that the Milky Way emits. On closer observation, the scientists involved realized something was a little off - while our galaxy stays together due to the gravitational pull of all the stars, planets, moons, etc., this galaxy didn’t have enough of those things to create the required gravitational force to hold it together. In other words, Dragonfly 44 has so few stars (and other objects) that they don’t produce enough gravity; if our galaxy had that few stars, they would simply drift apart. Which means, scientists hypothesize, that there must be mass that we can’t see creating the gravitational force to hold Dragonfly 44 together...and that mass is made up of dark matter.
Why is this so important?
While we’ve observed the effects of (what we assume is) dark matter for decades, scientists still have never seen it and have no idea what it is. They can sometimes map out where it must be, based on gravitational pulls and the way light bends around the unseen matter, but not being able to see it, know what it is, or even know exactly where it is makes it incredibly difficult to study. Dragonfly 44 offers scientists a rare opportunity to study dark matter in a large galaxy (rather than a small galaxy), where most of the mass is dark matter - they’re less likely to get confused by interfering stars or planets. And hopefully, this will lead to better knowledge of what dark matter is and thus, a better understanding of the Universe itself.
