M85-HCC1 is an ultracompact dwarf galaxy with a star density 1,000,000 times that of the solar neighbourhood, lying near the galaxy Messier 85. As of 2015, it is the densest galaxy known.
See also List of most massive galaxies
References
See also M59-UCD3 (second-densest galaxy known, as of 2015) M60-UCD1 (another dense galaxy)