The galaxy with 99.99% dark matter isn’t so mysterious any more

The dwarf galaxy known as Dragonfly 44 caused a stir recently: apparently it had way, way more dark matter than any other galaxy. Since this couldn’t be explained by our models of galaxy formation, it seemed like an oddball. But a new analysis reveals that Dragonfly 44 has much less dark matter than previously thought. In short: it’s totally normal.

Astronomers believe – based on quite a bit of evidence – that most of the matter in our universe is dark. It doesn’t interact with light. It doesn’t absorb light. It doesn’t emit light. It doesn’t even like light. This dark matter only makes its presence known through its gravitational attraction to all the normal matter in the universe.

While we don’t know what exactly the dark matter is made of, we’ve been able to successfully build models of the universe based on its existence. By dumping a load of dark matter into our theories, we’re able to reproduce observations of galaxies all across the universe.

Except for Dragonfly 44, an otherwise-boring dwarf galaxy sitting in the Coma Cluster. In 2016, astronomers reported a surprising result from this galaxy: it was apparently made of 99.99% dark matter. While dark matter is by far the dominant form of matter in the cosmos, it shouldn’t be this dominant. If the results held up, then this odd galaxy would have posed a serious challenge to our understanding of the universe: how the heck do you build a galaxy with that much dark matter?

It turns out, you don’t have to worry. It’s hard to directly measure the amount of dark matter in a galaxy (because it’s dark). The analysis in 2016 depended on counting the number of globular clusters orbiting Dragonfly 44, which is a loose-but-good-enough indicator of the amount of dark matter. They found around 80 globular clusters, indicating a supreme amount of dark matter.

But a recent, updated analysis accepted for publication by the Monthly Notices of Royal Astronomical Society has found only roughly 20 globular clusters, severely reducing the amount of inferred dark matter.

The new analysis brings the ratio of dark to normal matter to around 300:1, which while high isn’t out of the range of other galaxies, and perfectly allowed by our cosmological models.

When it comes to this particular “crisis” in cosmology, move along.