Astronomers have discovered a unique galaxy, from which dark matter appears to be completely missing.

We don’t really know what dark matter is and why it exists… or why it doesn’t. Image credits: NASA.

Dark matter, as the name implies, is a pretty mysterious thing. It appears to be an essential part of the universe, and yet we can’t observe it directly — we can only study its effects. Apart from gravity, dark matter doesn’t seem to interact in any way with regular matter, and physicists believe it comprises some kinds of elementary particles that have not yet been discovered

According to the latest astronomical models, dark matter appears to be ubiquitous, accounting for 26.8% of the mass of the universe (matter as we know it only accounts for 4.9%, and the rest 68.3% comes from dark energy — but let’s not get into that now). Despite it being so elusive, its effects have been identified through several different methods, and there is very strong evidence to suggest that dark matter does exist. Most astronomers take it as a given.

Not all galaxies are made the same when it comes to dark matter. For instance, dwarf galaxies have 400 times more dark matter than regular matter, whereas a galaxy like the Milky Way has about 30 times less. But this is the first time a galaxy that appears to have no dark matter was ever discovered.

This Hubble Space Telescope imaging of NGC1052–DF2 was obtained 2016 November 10, using the Advanced Camera for Surveys (ACS). Credits: van Dokkum.

This discovery is so bizarre that researchers aren’t really sure what to make of it. In an email to ZME Science, lead author Pieter van Dokkum explained that there are a few ideas as to how this type of galaxy might come to be, but none of them can truly explain things satisfactorily, and they’re not sure how to interpret this discovery.

“There are ideas out there for forming a dark matter-less galaxy, but none of them fit this particular object very well. For example, it could be that gas was pulled out of a big galaxy in a merger with another galaxy. That gas can then, in principle, contract and form a little galaxy (a “tidal dwarf”). However you wouldn’t expect that galaxy to be as big as this object (it’s almost the size of the Milky Way), and you’d also expect to see some other remnants around from the merger event. So we don’t really know!”

However, precisely because it is so unique and unusual, it might help astronomers solve some of the fundamental questions around the mysterious dark matter. It’s paradoxical, but the galaxy lacking dark matter could teach us a lot about dark matter itself.

Journal Reference: Pieter van Dokkum et al. A galaxy lacking dark matter. DOI 10.1038/nature25676

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