A recent research detailed in this paper suggests that our Galaxy (Milky Way), may have more complicated structure that we expected. First of all, instead of having a flat disc, the Milky Way has concentric ripples as shown at the picture below. Ripples can only be observed on our side of the Galaxy, due to the other half being obscured by stars and galactic centre. The scientists however expect this pattern to extend throughout the whole disc.
Secondly, if this is true and we extend this rippling pattern outside the assumed size of our Galaxy, we find that there are some over-densities of stars (Monoceros and Tri-Andromeda Ring) that match the ripples and thus they should be treated as part of the Galaxy itself. This would make the Milky Way around 50% bigger than it is considered to be now.
Thirdly, the rippling pattern can be an artefact of gravitational interaction with some other object: maybe another small galaxy or a body of a dark matter. Simulation to the left shows how this is possible - if a smaller object passes near the disc it can cause such ripples.