Disruption at the core of Astrophysics

Mystery galaxy DF-2 appears to lack the crucial ‘dark matter’ ingredient

The space and science-hunters expert website,, reports that a unique galaxy has been identified, which appears to contain record low levels of dark matter. For more than 40 years, so-called dark matter has been considered the scaffolding of the universe – a kind of infrastructure ‘glue’ that is known not only to play a key role in galactic evolution, but actually accounts for about 80 per cent of the mass of the universe. Yet now, astronomers have determined that the galaxy NGC 1052-DF2, or DF2 for short, has 400 times less dark matter than expected for an object of its size. 

Lead author of the newly-published research, Yale-based researcher Pieter van Dokkum, says that the discovery challenges the standard idea of how galaxies are born. While interactions between normal and dark matter have long been considered a key element in galaxy formation, the lack of dark matter in this galaxy appears to prove that this is not necessarily the case. He comments that: “Dark matter is apparently not a requirement for forming a galaxy. For a galaxy with a stellar mass of about 200 million solar masses, we expect a dark matter mass of about 80,000 million solar masses (galactic scale being measured in units the size of our own Sun, called ‘solar masses’). Yet the total mass of the system, however, weighs in at no more than 300 million solar masses, significantly less than we would anticipate.”

What does this mean in practice? Pieter van Dokkum and his colleagues realized that DF2 is about the size of the Milky Way, but has about 200 times fewer stars. It therefore belongs to a mysterious generic category known as the ‘ultra-diffuse galaxies’

These faint galaxies, which include DF2, can be as large as the Milky Way but shine only 1 percent as brightly. These ghostly, see-through galaxies is so diffuse that the galaxies behind them would theoretically remain visible.


Van Dokkum and his team have been viewing ultra-diffuse galaxies using the Dragonfly Telephoto Array, an instrument they custom-designed to investigate faint astronomical objects.

Without dark matter, how did the galaxy form?

The case for dark matter as a hidden substance is far from proven. There are a cogent raft of alternative theories for how galaxies are born. For example, Modified Newtonian dynamics (MOND) uses a modification of Newton’s laws to account for the discrepancies in visible and invisible mass, while Emergent Gravity suggests that space-time is made up of small elements whose collective motion produces the force of gravity.

As van Dokkum explains: “In those theories, dark matter is not real but an illusion, caused by our lack of knowledge of gravity on large scales. But if that’s the case, every galaxy should show a dark matter signature – it’s not something you can turn on or off in those models. So the very existence of a galaxy with virtually no dark matter makes the case that these non-dark-matter models are incorrect. Paradoxically, the absence of dark matter in this galaxy is evidence for dark matter’s existence.”

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