Scientists using the Hubble Telescope recently discovered an ultra-compact dwarf galaxy called M60-UCD1, which is the densest galaxy yet discovered. The closest star next to our sun is Alpha Centauri, which is 25.6 trillion miles away. In M60-UCD1, there are about 10,000 stars in that same span, spaced at about the distance from Earth to Pluto. The concentration of stars in that galaxy is about 15,000 times greater than in our corner of the Milky Way. The presence of a supermassive black hole in its center suggest that it was probably part of a much bigger galaxy at one point and much more similar to our Milky Way.