This image by one of the European Southern Observatory's ground telescopes shows off many thousands of distant galaxies, and more particularly a large group belonging to the massive galaxy cluster known as Abell 315. As crowded as it may appear, this assembly of galaxies is only the proverbial "tip of the iceberg", as Abell 315 -- like most galaxy clusters -- is dominated by dark matter, researchers said. Galaxy clusters are some of the largest structures in the universe held together by gravity. Galaxies in these giants contribute to only 10% of the mass, with hot gas in between galaxies accounting for another 10%. The remaining 80% is made of an invisible and unknown ingredient called dark matter that lies in between the galaxies.