NASA-International Space Station

In a new research that involved the mapping of the shape of the galactic dark matter, astronomers have determined that the halo of dark material surrounding our Milky Way Galaxy is shaped something like a gigantic, compressed cosmic beach ball.

This result is significant because it is the first time that the three-dimensional shape of an individual dark-matter halo has been convincingly measured.

Dark-matter haloes account for over 70 percent of the mass in galaxies such as the Milky Way, but this dark matter is hidden; all we see when we look up in the sky is the small amount of stars and gas sitting in the centers of these haloes.

It might not be probable to detect it through normalmeans, but dark matter obeys the laws of gravity and tugs on small dwarf galaxies as they orbit around the Milky Way.

By observing the orbits that these dwarf galaxies follow, astronomers can infer where the dark matter must be using Newton's law of gravity.

While it would take approximately a billion years to watch a typical dwarf galaxy orbit just once around our home galaxy, dwarf galaxies get shredded by tidal forces as they orbit the much more immense Milky Way and leave stars like breadcrumbs along their path.

Using observations of such tidal debris from a dwarf known as the Sagittarius Dwarf Galaxy, astronomers have been able to rebuild the orbit of Sagittarius and derive models for the Milky Way and its dark-matter halo.

But, these models had met an impasse, as diverse parts of the orbit suggested wildly different solutions.

In September 2009, Law and colleagues Majewski and Johnston recommended a solution. By allowing the dark matter halo to be triaxial - that is, to have diverse axis lengths in all three dimensions - it is probable to fit the entire orbit of Sagittarius simultaneously.

The planned solution suggests that the invisible dark-matter halo of the Milky Way can be visualized as some kind of cosmic 'beachball' that has been compacted sideways.

The fact that the 'beach ball' was compacted from the side came as a surprise.
It suggests that the dark matter halo and the disk of stars in the Milky Way are tilting roughly perpendicular to each other.

"We expected some amount of flattening based on the predictions of the best dark-matter theories, but the extent, and mainly the orientation, of the flattening was quite unexpected. We're pretty thrilled about this, because it begs the question of how our galaxy formed in its present orientation," said Law.