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Galaxies Rotate in Sync, Raising Dark Matter Questions

The universe is filled with galaxies, and often a large galaxy like our own will have several smaller ones orbiting it. Astronomers looked at one particular group of galaxies and noticed their circling was a bit too orderly for current models to explain.

Most scientists’ understanding of our universe includes a substance called dark matter, which accounts for 80 percent of the matter in the universe. Dark matter was first hypothesized in order to account for the rotation of galaxies, which didn’t seem to have enough conventional matter to keep them from flying apart like a smoothie in a lidless blender. Dark matter provides the extra stuff needed to keep galaxies together and was likely involved in galaxy formation. Dark matter appears to be cobwebbed across the universe. Scientist suspect that dwarf galaxies form along these dark matter threads and converge where they meet, merging into larger galaxies.

Under this framework, satellite galaxies should be distributed randomly around their host, following elongated orbits in arbitrary directions. This assumption was challenged when scientists found that satellites of our own Milky Way and the Andromeda galaxy, our closest major neighbor, don’t follow this prediction. The small companion galaxies appear to rotate in sync with each other, following fairly circular paths in a disk-shaped plane around their host galaxy. [Gallery: 65 All-Time Great Galaxy Hits]

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