|MadSci Network: Astronomy|
The word "biggest" could mean either the galaxy with the largest size (in kilometers) or the largest mass (in kilograms).
In either case, the biggest galaxies are called cD galaxies. These exist in the centers of large clusters of galaxies, and are formed by the merger of many smaller galaxies.
Big cD galaxies can have 100 times the mass of a galaxy like the Milky Way, and they can emit 10-20 times as much light as our galaxy does.
A good example of a cD galaxy is the giant galaxy M87.
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