Even though scientists have managed to quantify how much dark matter lurks in distant galaxies, astronomers have been hard-pressed to figure out how much of the mysterious stuff lies within our own.
But in a paper published in the journal Nature Physics, a team of researchers has managed to measure the amount of dark matter in the inner Milky Way, which could shed light on the structure and evolution of our galaxy, and perhaps of others as well.
Dark matter is that mysterious stuff that accounts for 84.1% of the matter in the universe, while normal matter – all the stuff we can see, from galaxies to stars and planets and black holes and us – makes up a mere 15.9%*. Because it doesn't interact with light, it is completely invisible to us, and the only way to tell it's there is by looking for its powerful gravitational influence on the normal matter around us.
For example, astronomers are able to calculate how much dark matter is in far-off galaxies by looking at their spin. Basically, astronomers can tell how much mass is in a spiral galaxy by watching how fast it rotates. The faster the spin, the more massive the galaxy. And if they're more massive, the galaxies should be brighter, too, because they should be filled with more stars.
But astronomers noticed galaxies that were spinning really fast, even though they weren't bright. If the mass causing these galaxies to spin so fast wasn't from stars or gas, then what exactly was it?
In the many decades since the discovery of this so-called dark matter, the question hasn't been answered yet. But whatever dark matter is, scientists need to measure how much of it there is in a given galaxy in order to understand the behavior of galaxies and the large-scale structure of the cosmos. For distant galaxies, that's relatively easy to do.
Ironically, for the galaxy we live in, that's a much more difficult measurement to make, said UC Irvine astrophysicist James Bullock, who was not involved the study.
"[Measuring] anything is hard when you're inside of it," Bullock said. "It's kind of like trying to figure out what kind of house you live in without ever leaving your house."
To further complicate matters, dark matter isn't sprinkled evenly throughout a given galaxy. There's a ton of it on the outer edges of a galaxy, in a huge invisible halo. In the outer parts of a galaxy, dark matter can outnumber normal matter on a roughly 8-to-1 ratio. But closer to the bright heart of a galaxy, there are so many stars that the normal matter overwhelms the dark matter present.
In the inner Milky Way, Bullock said, "it's very hard to tease out how much dark matter there is, because the contribution from normal matter is pretty high."
Stuff in the Milky Way doesn't all travel at the same speed, but varies with its distance from the galactic center, said lead author Fabio Iocco, an astroparticle physicist at the ICTP South American Institute for Fundamental Research in Sao Paulo, Brazil.
"It's not like a solid disk, it's like a disk made of several onion rings," Iocco said of the Milky Way, "and each ring is spinning at a different velocity."
Many of the measurements of speed in the inner Milky Way rely on some marker, tracking targets such as masses of hydrogen gas as they move around, or star clusters as they travel through space. But models based on one of these tracers don't necessarily capture the overall motion and velocity of stuff rotating around the galaxy's center.
To get a better bead on the speed, Iocco and his colleagues tracked the motion of a wide variety of different markers, such as hydrogen gas, carbon monoxide, the giant molecular clouds of star-forming regions and planetary nebulae, and used these tracers in combination to come up with a more accurate picture of the galactic spin speeds.
"When we started working on it, we were a little surprised that nobody had done this calculation," Iocco said.
Sure enough, the researchers found their markers were moving so fast that there must be some dark matter contributing to the mix – at most, the dark matter would roughly equal the amount of normal matter in the inner Milky Way. That's far less than the overall Milky Way, where dark matter is several times more abundant than normal matter.
The findings, Iocco said, "may help us understand some things about galaxy formation in general."
The findings could help astronomers to better understand how to search for dark matter in our own galaxy, Bullock said.
"People are trying to search for dark matter passing through the Earth all the time," Bullock said, "and the better we know the local density of dark matter, the better we can then tune those experiments to look for the signal."
The more accurate estimates for the Milky Way could also help scientists understand the structure of those distant galaxies, too.
"It's amazing what we don't know about our own galaxy," Bullock said. "And if we knew it would be great, because it would help us use our own galaxy as a Rosetta stone to help translate other systems."
*While dark matter makes up 84.1% of all matter, it's just 25.9% of the overall mass-energy density of the universe, according to the latest results from ESA's Planck satellite. The vast majority of the mass-energy density, a whopping 69.2%, is filled with dark energy, the strange repulsive force that's causing the universe to expand. All the normal matter in the universe, known as baryonic matter, makes up a tiny 4.9%.