From way out in space, our home galaxy would look pretty much like a flat disc of bright stars with spiral arms. But the Milky Way is actually surrounded by a vast halo of invisible dark matter that's shaped like a squashed beach ball.
That's according to a new map of our galaxy's dark matter announced at a meeting of the American Astronomical Society in Washington, D.C.
Even though dark matter cannot be seen, scientists think that it must be present because they can see the effects of its gravity as it pulls on the orbits of stars and galaxies.
"While we may not be able to see the dark matter, it's a vitally important component of our galaxy. It makes up the bulk of what the actual stuff is in our galaxy," says David Law, an astrophysicist at the University of California, Los Angeles, who says dark matter accounts for more than 70 percent of the mass in galaxies like the Milky Way.
Law wondered what the cloud, or halo, of dark matter enveloping the Milky Way would look like if you could actually see it. He knew he could get clues about its shape from how the dark matter alters the paths of dwarf galaxies and clusters of stars as they orbit the Milky Way.
"Based on the shape of those paths, we can say what the shape of the dark matter itself has to be," he says.
Now, you can't actually watch a dwarf galaxy go all the way around our Milky Way — that would take about a billion years. But Law and his colleagues used a kind of trick to reconstruct the path of one orbiting galaxy.
"As it flies around the Milky Way, tidal forces from the Milky Way are shredding stars out of it into streams that trail behind it and even go ahead of it, along its orbit," Law says.
These streams of stars are like a trail of breadcrumbs showing the orbiting galaxy's path around the Milky Way, he says. From that, he and his colleagues could figure out the shape of the big halo of dark matter that surrounds our galaxy.
Picture the bright, visible part of our Milky Way as a flat, round, dinner plate, says Law, and imagine it sitting inside a large beach ball, which represents the dark matter halo.
"If you then came along to that beach ball and put one hand on one side and one hand on the other and squeezed it, that's something like we think the dark matter is," Law says. "It's squeezed along that direction, and the dinner plate of the disk of the Milky Way is sitting inside of it."
This new map should help scientists as they try to understand the nature of dark matter. At the moment, there are lots of theories about what dark matter might be, but nobody knows.
Whatever dark matter is, says Law, it's got to be able to form this kind of squashed beach ball shape. "You have to have some kind of a dark matter particle which can build a halo like this, because this is generally what we see surrounding the Milky Way."