Dark matter is believed to make up more than 80 percent of all matter in the universe, but what it actually is remains a mystery.
Now, astronomers have found something that gives us a major clue.
In a pair of new studies published in Nature Astronomy and Monthly Notices of the Royal Astronomical Society, the astronomers report that they’ve found a low mass object in the ancient outskirts of the cosmos by examining the gravitational distortions in the light of a much larger galaxy.
This oddity, they claim, is the lowest mass object ever found using this technique, which is called gravitational lensing.
“It’s an impressive achievement to detect such a low mass object at such a large distance from us,” Chris Fassnacht, an astronomer at the University of California, Davis, and co-author of the Nature Astronomy study, said in a statement about the work. “Finding low-mass objects such as this one is critical for learning about the nature of dark matter.”
Still, it’s chunky: the object weighs more than a million solar masses, meaning more than a million times the weight of the Sun. Residing some 10 billion light years away, we’re observing it when the universe was only 6.5 billion years old, or less than half its current age.
Despite those mind-boggling proportions, this is the lowest mass object ever found using gravitational lensing, according to the authors — by an impressive factor of around 100.
To uncover the object, the researchers combined the observations of radio telescopes around the globe, including the Very Long Baseline Array (VLBA) in Hawai’i, and the European Very Long Baseline Interferometric Network (EVN) in Europe, to create an “Earth-sized super-telescope,” as the researchers described it. Then, they had to develop purpose-built algorithms to reveal the object in the mountains of data that yielded.
Since dark matter is invisible, we can’t observe it directly. But we can observe the pull of its gravity on other objects. Its gravitational influence, in fact, is believed to be instrumental to the existence of the largest structures in the cosmos, pulling ordinary matter around “halos,” or clumps of itself, to form galaxies and perhaps stars.
That’s the prevailing theory, anyway, described in the leading cosmological framework, the lambda CDM model — CDM being short for “cold dark matter.” While there’s many dark matter candidates, a lot of the debate focuses on whether dark matter is hot or cold. Hot dark matter would be made of lighter, fast moving particles, and cold would be made of heavier and slower ones. If it’s cold and slow, it would also be “clumpy,” clotting into halos instead of being smoothly spread out across the cosmos.
The discovery of this newest dark object “is consistent with the so-called ‘cold dark matter theory’ on which much of our understanding of how galaxies form is based,” Devon Powell at the Max Planck Institute for Astrophysics, the lead author of the Nature paper, said in the statement. If dark matter is indeed clumpy, then we should be spotting halos of various sizes all throughout the universe.
“Having found one,” Powell added, “the question now is whether we can find more and whether their number will still agree with the models.”
There’s still some light to shed on this dark object, however. The astronomers haven’t confirmed what it is yet; the favored explanation is that it’s a halo of dark matter, but it also could be an inactive but ultra-compact dwarf galaxy. Still, that it’s been spotted at all is an achievement in itself. “The precision measurement of its mass, size and position is unprecedented for an object in this mass range at this distance,” the authors wrote in the Nature study.
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