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Two collisions with a dwarf galaxy over the last nearly 2 billion years may have been the cause of the Milky Way's spiral arm structure, scientists say. The new findings hint that impacts with even relatively small galaxies have played an important role in shaping galactic structure throughout the universe, researchers said. In trying to explain the shape of our own galaxy, the Milky Way, with its prominent spiral arms rooted in a central bar, scientists have traditionally dismissed the influence of outside forces, even though astronomers have seen shape- changing mergers of other galaxies.