Galaxy mergers
How do you form a large galaxy? Does it just start out as a big galaxy or do you build it up by colliding and merging little galaxies? Numerical simulations that attempt to model large parts of the Universe seem to show that larger galaxies can be created from the mergers of dwarf galaxies in a "bottom up" approach. Of course they are just simulations, so is there evidence that that is actually happening? The short answer is yes. A nice example is in a new image just released by the Hubble Space Telescope (HST) showing a galaxy undergoing this very process of eating smaller galaxies and putting on the lightyears around its middle.

Composite of separate exposures made by the ACS on the HST. It shows the Spiderweb Galaxy at the centre surrounded by hundreds of other galaxies. CREDIT: NASA, ESA, George Miley and Roderik Overzier (Leiden Observatory, the Netherlands)