What is the heaviest galaxy in the universe?
The largest (and most massive) galaxy that we know of is the galaxy IC 1101 at the center of the cluster Abell 2029.
In general, the most massive galaxies are the elliptical galaxies at the center of galaxy clusters, known as BCGs (or Brightest Cluster Galaxies). In a cluster of galaxies, galaxies tend to fall into the center, making a very massive galaxy at the core.
There could be larger galaxies in the Universe, IC 1101 is just the largest we have found so far.
IC1101 is the big, fuzzy galaxy at the center of this image.
Image of IC 1101, taken by the Hubble Space Telescope.