The Sun and Proxima Centauri and most of the stars you've heard of orbit the centre of the Milky Way galaxy like children peacefully riding a carousel (with some weirdness caused by dark matter that we don't need to get into). Now, imagine if a few toddlers were sprinting and shrieking across the peaceful scene. Who sent these nightmares? In the case of stars, scientists think they could have come from another galaxy.