A lost galaxy called 'Loki' may be hiding inside the Milky Way

The Milky Way galaxy grew into its current form with the help of smaller galaxies over time, which it has "consumed" or merged with. Astronomers are able to pick out which stars in the Milky Way came from other galaxies by identifying certain features, like the eccentricities of their galactic orbits and how many heavier elements they contain. Properties of some of the merged galaxies can then be determined when astronomers find collections of stars with similar features.