Explanation: Big, beautiful NGC 5584 is more that 50,000 light-years across and lies 72 million light-years away toward the constellation Virgo. The winding spiral arms of this gorgeous island universe are loaded with luminous young star clusters and dark dust lanes. Still, for earthbound astronomers NGC 5584 is not just another pretty face-on spiral galaxy. Home to some 250 Cepheid variable stars and a recent Type Ia supernova explosion, key objects for astronomical distance determinations, NGC 5584 is one of 8 galaxies used in a new study that includes additional Hubble Space Telescope observations to improve the measurement of Hubble’s Constant – the expansion rate of the Universe. The results of the study lend weight to the theory that dark energy really is responsible for accelerating the expansion of the Universe, restricting models that try to explain the observed acceleration without the mysterious dark energy. In this sharp Hubble image of NGC 5584, many of the small reddish smudges are distant background galaxies.