Our Milky Way galaxy is a typical spiral galaxy, about 100,000 light years across, and with about 10¹¹ stars. However, it is also quite interesting in that it has a bunch of satellite galaxies around it, arranged in the same plane as the galaxy! Cosmological theory predicts that these galaxies should occupy a large, nearly spherical halo, but the pancake-like structure has baffled scientists for a long time. Now, finally, the puzzle has perhaps been solved.

Milky Way (Courtesy: CNN)All galaxies have satellite galaxies, which inhabit pockets of dark matter. Dark matter does not interact with light and the only way that we can infer its existence is by detecting the gravitational influence it exerts on normal matter, such as stars. Soon after the Big Bang, cold dark matter formed the universe’s first large-scale structures, which then collapsed under their own weight to form vast halos. Normal matter was attracted to these halos, and galaxies were formed. But, this should lead to a central large galaxy, surrounded by smaller galaxies arranged in a sphere. But Milky Way's satellites are arranged in a flat circle!
Scientists from University of Durham simulated the evolution of parts of the universe, randomly selected from a large cosmological volume, using a sophisticated supercomputer model. They carried out six simulations, and found that galaxies must end up in such a pancake-like distribution:). With this research, an important puzzle has been solved, and a coherent picture of how galaxies like the Milky Way emerged from the Big Bang is now beginning to fall into place.
More research and simulations will try to understand the formations of even larger galaxies, and perhaps evolution of galactic clusters as well.