Astronomers are mapping an unseen ocean of dark matter swirling out of the Milky Way.

A mysterious trail of stars, agitated by a small galaxy that will collide with the Milky Way, may be on the verge of unraveling the mysteries of dark matter.

The trail of stars, located outside the star-flecked spiral arms of the Milky Way's central disk in a region called the galactic halo, is being carried along in the cosmic slipstream of a dwarf galaxy in orbit around the Milky Way, according to a new sky map created by astronomers.

The galaxy called the Large Magellanic Cloud (LMC), is in orbit about 130,000 light-years from Earth and causes the wake of cosmic matter behind it. At first glance, the LMC track seems to be composed solely of stars, but researchers know that the stars are right along the course. They are suspended in a much larger and completely unseen presence.

Astronomers are interested in this ripple in space because they think it could be made up of dark matter — the mysterious non-luminous substance making up the vast majority of matter in the universe. Predictions say that dark matter, invisible and interacting with matter that we can only see by gravity, should be all around the galactic halo.

"We think this wake is made up of dark matter, and it drags stars along with it, which is how we can detect it," study co-author Nicolás Garavito-Camargo, a University Arizona doctoral student, said in a statement.

Dark matter's gravitational influence can be observed throughout the universe: It is our galaxy's vital scaffolding, gluing stars and planets to it so they don't fly off as the galaxy spins. Yet what exactly dark matter is, or how it behaves, is one of the greatest mysteries of astronomy. The researchers hope that by studying the wake, they will be able to study dark matter, which they consider being the vast majority.

If the wake's stars are like leaves floating on a dark matter pond, the way that the leaves are disturbed by a boat (in this case, the LMC) can tell us a lot about the pond itself.

"You can imagine that the wake behind a boat will be different if the boat is sailing through water or honey," said lead author Charlie Conroy, a professor of astronomy at Harvard University. "In this case, the wake properties are determined according to what dark matter theory we apply."

The group has used their new map and the position of the wake to confirm a theoretical model, created by another group of researchers, about just how dark matter should be distributed across the galactic halo; they are now running tests to see which of the theories about dark matter best fits the wake's shape and location.

This map, produced from telescope data from NASA and the European Space Agency (ESA), also provides essential information about the violent future of our galaxy. As the LMC orbits the Milky Way, the gravitational tug from the dark matter in the Milky Way’s galactic halo is slowing it down, sending the LMC into smaller and smaller orbits. The LMC will continue to move closer to the Milky Way until around 2 billion years from now the two collide.

The fusion of two galaxies is a surprisingly commonplace event across the universe. The Milky Way probably merged with a small galaxy 8 billion years ago, and galaxy mergers are one of the main reasons for the growth of all great galaxies.

"This robbing of a smaller galaxy's energy is not only why the LMC is merging with the Milky Way, but also why all galaxy mergers happen," study co-author Rohan Naidu, said a graduate student at Harvard University. "The wake in our map is a really crisp confirmation that our basic picture for how galaxies fuse is about to happen."

The researchers published their conclusions on 21 April in the journal Nature.

Original publication on Live Science.