The ultra-compact dwarf galaxy, dubbed M60-UCD1, was found in what's known as the Virgo cluster of galaxies, researchers said.
Imagine the distance between the Sun and the star nearest to it - Alpha Centauri. That's a distance of about 4 light years. Now, imagine as many as 10,000 of our Suns crammed into that relatively small space.
That is about the density of a galaxy discovered by an international team of astronomers led by a Michigan State University faculty member.
"This galaxy is more massive than any ultra-compact dwarfs of comparable size and is arguably the densest galaxy known in the local universe," said Jay Strader, MSU assistant professor of physics and astronomy.
The galaxy was discovered in the Virgo cluster of galaxies, a collection of galaxies located about 54 million light years from our own Milky Way.
What makes M60-UCD1, so remarkable is that about half of its mass is found within a radius of only about 80 light years. This would make the density of stars about 15,000 times greater than found in Earth's neighbourhood in the Milky Way.
"Travelling from one star to another would be a lot easier in M60-UCD1 than it is in our galaxy. Since the stars are so much closer in this galaxy, it would take just a fraction of the time," Strader said.
The discovery of ultra-compact galaxies is relatively new - only within the past 10 years or so. Until then, astronomers could see these "things" way off in the distance but assumed they were either single stars or very-distant galaxies.
Another intriguing aspect of this galaxy is the presence of a bright X-ray source in its centre. One explanation for this is a giant black hole weighing in at some 10 million times the mass of our Sun.
Astronomers are trying to determine if M60-UCD1 and other ultra-compact dwarf galaxies are either born as really jam-packed star clusters or if they are galaxies that get smaller because they have stars ripped away from them.
The possible massive black hole, combined with the high galaxy mass and Sun-like levels of elements found in the stars, favour the latter idea.
A giant black hole at the centre of M60-UCD1 helps tip the scales against the scenario where this galaxy was once a star cluster, since such large black holes are not found in these types of objects.
The galaxy was discovered using NASA's Hubble Space Telescope. Follow-up observations were done with NASA's Chandra X-ray Observatory and ground-based optical telescopes, including the Keck 10-metre telescope in Hawaii.
The study was published in the Astrophysical Journal Letters.