A new study has found galaxies with more neighbors tend to be larger than their counterparts that have a similar shape and mass, but reside in less dense environments. The team, which used a machine-learning algorithm to analyze millions of galaxies, reports that galaxies found in denser regions of the universe are as much as 25% larger than isolated galaxies. The findings resolve a long-standing debate among astrophysicists over the relationship between a galaxy’s size and its environment, but also raise new questions about how galaxies form and evolve over billions of years.