Mysterious force tearing into galaxy
We have a bullet hole. But there's no smoking gun. Something has torn a hole through the Milky Way. A dark something. Something so strange it could rewrite the astronomy books.
The American Physical Society annual meeting has been presented with evidence of a 'dark impactor' - an invisible source of gravity - tearing holes in our galaxy's tidal star formations.
Harvard-Smithsonian Center for Astrophysics researcher Dr Ana Bonaca was studying stellar streams, long lines of stars stretched out and flowing through our galaxy.
But the longest of these stellar streams, dubbed GD-1, looked a little odd.
It should have been a single line, stretched across the sky by the galaxy's gravity. But there's a monstrous hole in the star stream. And that hole - like a bullet hole - has a jagged edge where the force of an impact knocked stars out of alignment.
There are "streams'' of stars orbiting our galaxy, the Milky Way. They once belonged to 'globular clusters', small nearby galaxies that have been drawn into the Milky Way by its overwhelming gravitational attraction.
"So what's new here?" says Monash University associate professor in astrophysics Dr Michael Brown. "Well this stream has notches and twists in it, which suggest its gone by something several million times the mass of the Sun."
Attempts to track down the cause of the hole have failed.
Which is odd, as the celestial collision seems to have happened relatively recently. And whatever it was is likely to have been very big.
"It could be that it's a luminous object that went away somewhere, and it's hiding somewhere in the galaxy," Dr Bonaca told LiveScience.
But it also could be something astronomy as been seeking evidence of for decades: dark matter.
The universe is held together by gravity. We understand how it works, just not exactly what it is. An object gives off an attracting force. The more mass, the greater the attraction.
Problem is, there's more gravity about than there is observable mass.
In fact, there's must be about 27 per cent more mass out there somewhere than we can see in the form of stars, dust and gas.
It's been dubbed 'dark matter' simply because it's invisible. And we've not been able to figure out what it is made of.
But this hole could give us some clues as to what form such 'dark matter' takes.
The hole in the stream of stars is about 30-65 light years across.
That's the size of a globular star cluster.
Did a dense ball of 'dark stars' recently blast its way through our galaxy?
Dr Bonaca isn't sure. She says the cause could still be a more mundane 'luminous' object, or even a supermassive black hole. It's just that we have no idea how fast it was moving.
But the possibility that it was a clump of dark matter remains tantalising.
"Cold dark matter theory predicts that there could be clumps of unseen dark matter orbiting within the Milky Way, which could be responsible," Dr Brown says.
"And before anyone gets worried, even these clumps shouldn't be dense enough to damage the Solar System."
It's not exactly evidence dark matter is real. Not yet.
But it could indicate the missing source of interstellar gravity is not evenly distributed.
It may be 'lumpy'. It may be scattered in irregular clumps across galaxies - in much the same way stars are. And that would rule out some of the possibilities being considered for the composition of this mysterious substance.
If the culprit was a black hole, it would have to be enormous - on the same scale as the supermassive black hole found at the centre of most galaxies.
But usually most galaxies only have one. And there's as yet no sign of an accretion disk of plasma, dust, gas - or even orbiting stars.
"This is a really interesting paper and the results are consistent with theory, but it certainly isn't a slam dunk yet," Dr Brown says. "Indeed, the paper itself outlines future research that could further test this really interesting result."
Dr Bonaca's presentation to the society is yet to be published in a peer-reviewed science journal, though a preprint is available online.