Using a century-old technique to probe the borders of our galaxy, a team of Australian astronomers has estimated the amount of dark matter in the Milky Way. Intriguingly, according to their new calculations, there could be half as much of this poorly understood substance in our galaxy as once thought. Their work has been published in The Astrophysical Journal.
Dark matter is an elusive substance; it doesn’t absorb, reflect or emit light, meaning it’s invisible to both our eyes and the instruments we use to detect normal matter. Because it doesn’t interact with the electromagnetic force, we only know it exists because it exerts gravitational effects on visible matter. Although we can’t see it, physicists have calculated that it makes up around 25% of the universe. The stuff of our bodies, stars, dust and planets, or “normal” matter, makes up a mere 4%. The rest is something even more strange- dark energy.
Although dark matter is very difficult to study, researchers are able to measure its mass using a method pioneered in the early 19th Century by a British astronomer called James Jeans. This technique, which was developed long before scientists even knew dark matter existed, involves measuring the speed that stars are travelling throughout our galaxy. While astronomers have been doing this for some time, they had never used it to examine the very edges of the Milky Way, which is what they did in this latest study.
By peering this far out, some 5 million billion kilometers from Earth, they were able to obtain measurements that allowed them to calculate the mass of dark matter in our galaxy. Although it was found to be a pretty huge figure, some 800,000,000,000 times the mass of our sun (8 x 1011 solar masses), this is around half of the previous estimates.
The team’s new measurement has also helped them solve a problem that has been “a thorn in the cosmological side for almost 15 years,” according to study co-author Professor Geraint Lewis.
As explained by lead author Dr. Prajwal Kafle, the widely accepted idea of galaxy formation and evolution -- the Lambda Cold Dark Matter theory-- predicts that there should be numerous large satellite galaxies around the Milky Way. However, when their new measurement is applied, the theory predicts that there should only be three. And that is precisely what we observe: the Large Magellanic Cloud, the Small Magellanic Cloud and the Sagittarius Dwarf Galaxy.