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We do not know what it is. We do not even know if it's an ordinary question – but we know that Something blew a series of holes through stars of the Milky Way.
"It's a dense ball of something," said Ana Bonaca, a researcher at the Harvard-Smithsonian Astrophysics Center, who discovered evidence of the impactor.
Bonaca analyzed a series of stars called GD – 1 – a very long and thin stream of stars from the Milky Way. The GD-1 stars have been studied since their discovery in 2006, and Bonaca is using the recently released Gaia telescope data to analyze them in more detail and find something odd in the middle of the creek.
This type of stellar flow is created by the (gravitational) tidal force of the Milky Way, which bends and expands the flow, creating an empty space about halfway through the flow.
But when Bonaca looked at GD-1 more recently, she discovered a second hole – and another weird. The second gap is not smooth like the first, but has an irregular edge, as if something had pulled on it.
"It's a dense ball of something," Bonaca said.
The "ball" should be something absolutely massive, much bigger than a star and more massive than any black hole except the biggest one. It is not out of the question that a supermassive black hole is the culprit, but if that were the case, it would be necessary to have one at the scale of the supermassive black hole located at the center of our galaxy. There is no clear reason why such a black hole would exist near the edge of our galaxy, and astronomers did not see any effects.
This leaves another tantalizing possibility: a massive object in black matter.
Dark matter is a hypothetical form of matter that accounts for about 85% of the matter in the universe and about a quarter of its total energy density. We do not know what dark matter is and we have never seen it – but we have seen its effects and astronomers are convinced that it exists. We also have no idea how dark matter could be distributed in the universe – is it thin and diffusive, or bulky and bushy? If dark matter was actually projected through the GD-1 stars, this would suggest the latter. However, a big ball of dark matter is still speculative at this point, although it seems to align quite well with the evidence.
Results have not yet been peer-reviewed, although they have been positively received at the American Physical Society conference in Denver, where they were presented.
At this point, the eventful history of GD-1 stars is not yet sufficiently established to draw a definitive conclusion. Anyway, is Bonaca's hypothesis true? Something has pulled a huge "bullet" into our galaxy – and we do not know what it is.
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