M60-UCD1 is an ultracompact dwarf galaxy. Half of its stellar mass is in the central sphere only 160 light years in diameter. It is probably the densest known galaxy with over one hundred stars per cubic light-year, and it is the smallest and least massive galaxy known to host a central black hole. It is also the most massive ultracompact dwarf galaxy known. It is also one of the oldest galaxies in the universe.
Image Credit: NASA