3 April 2018: The Mystery of the Missing Dark Matter

In the news today: you could be shaking your head about political antisemitism, worrying that the Chinese-American trade slap-fight could be a practice run for a more lethal form of war, or hovering your finger over the delete-my-facebook-account button.

Or, you could turn away from the front page of doom and ponder The Mystery of the Missing Dark Matter, which is bamboozling astronomers around the globe this week. 

They’re stumped by an object called NGC1052-DF2, a large galaxy 65 million light years away. It’s unusually diffuse: it’s bigger than our own Milky Way galaxy, but much less massive. It also seems to be missing all of its dark matter — which is truly perplexing, because dark matter is by far the largest component of every other galaxy ever studied.

And the brilliant twist in this astronomical mystery? This absence of Dark Matter is evidence that it actually exists.

OK, first: what’s Dark Matter? The stars and planets, gas and dust we see inside galaxies across the universe are made of the same materials we find here on Earth: everyday, garden variety matter.

Astronomers have also found lots of evidence for mysterious stuff they call Dark Matter. It’s actually by far the most plentiful matter in the universe — maybe five times more than the matter we can actually see.

But Dark Matter is, well, dark: we can’t see it with telescopes that detect visible light, radio waves, X-rays, UV, or any kind of radiation.

We know it’s there because Dark Matter still affects everything around it through the force of gravity. Here’s an example: astronomers can measure the speed of stars in a galaxy as they orbit the central core. Out towards the galaxy’s edge the stars seem to be moving way too fast; there isn’t enough visible material in the galaxy to keep them in orbit at those speeds.

The best explanation is that there has to be a huge amount of Dark Matter clumped in and around the galaxy as well — much more than the visible stars and gas.

The story is the same for every galaxy astronomers have ever studied. Every galaxy, that is, except NGC1052-DF2. 

The journal Nature this week has a report by an international collaboration of astronomers who studied that galaxy with a slew of instruments, including the Hubble Space Telescope. They’ve concluded that this one weird galaxy contains almost no dark matter at all.

Now, you might think that the lack of dark matter here would suggest that perhaps this mysterious material doesn’t actually exist. But if you assume dark matter isn’t real, then you need some other explanation for those high rotational speeds in all the other galaxies. For example, maybe we don’t properly understand how gravity works over long distances. 

But if that was the case, then that different gravitational force law would apply to NGC1052-DF2 as well: it’s outer stars should be whipping around at high speed. The astronomers looked, and they’re not: they found stars moving so slow, they’re essentially standing still.

The best explanation is that dark matter does in fact exist; it’s just absent in this one strange galaxy.

If astronomers can find more galaxies like NGC1052-DF2, they might yet solve the mystery of the missing Dark Matter. Perhaps it was stolen during a collision with another galaxy long ago.

Or perhaps this galaxy formed differently from other galaxies — and the missing Dark Matter was never there to begin with.

Original paper by van Dokkum et al in Nature, at www.nature.com/articles/nature25767 

Hubble Space Telescope images of the mysterious galaxy: hubblesite.org/news_release/news/2018-16