Question: what is the largest known galaxy

Guy Rixon answered on 14 Mar 2018:

Galaxies usually group together in clusters (e.g. and see the wikipedia article on galaxy clusters). The larger galaxies in the cluster tend to “eat” the smaller ones, so that one super-galaxy forms near the centre of the cluster. It’s called a “1st-ranked cluster galaxy” in the science literature and is usually an “elliptical” galaxy meaning that it doesn’t have a disc and spiral arms (these are thought to get washed away in the mergers).
These 1st-ranked galaxies are the largest known. They seem to grow to roughly the same size and brightness, somewhat larger than our own galaxy. I don’t know which cluster has the biggest, but you might be able to find out by googling.
Our galaxy is in a small cluster, with the Andromeda galaxy and some smaller galaxies. In the far, far future, our galaxy will merge with Andromeda and form (probably) a large elliptical galaxy.