There's more to the cosmos than meets the eye. About 80 percent of the matter in the universe is invisible to telescopes, yet its gravitational influence is manifest in the orbital speeds of stars around galaxies and in the motions of clusters of galaxies. Yet, despite decades of effort, no one knows what this "dark matter" really is. Many scientists think it's likely that the mystery will be solved with the discovery of new kinds of subatomic particles, types necessarily different from those composing atoms of the ordinary matter all around us. The search to detect and identify these particles is underway in experiments both around the globe and above it.Scientists working with data from NASA's Fermi Gamma-ray Space Telescope have looked for signals from some of these hypothetical particles by zeroing in on 10 small, faint galaxies that orbit our own. Although no signals have been detected, a novel analysis technique applied to two years of data from the observatory's Large Area Telescope (LAT) has essentially eliminated these particle candidates for the first time. See: Fermi Observations of Dwarf Galaxies Provide New Insights on Dark Matter 04.02.12
Dwarf spheroidal galaxy (dSph) is a term in astronomy applied to low luminosity galaxies that are companions to the Milky Way and to the similar systems that are companions to the Andromeda Galaxy M31. While similar to dwarf elliptical galaxies in appearance and properties such as little to no gas or dust or recent star formation, they are approximately spheroidal in shape, generally lower luminosity, and are only recognized as satellite galaxies in the Local Group.
While there were nine "classical" dSph galaxies discovered up until 2005, the Sloan Digital Sky Survey has resulted in the discovery of 11 more dSph galaxies—this has radically changed the understanding of these galaxies by providing a much larger sample to study.
Recently, as growing evidence has indicated that the vast majority of dwarf ellipticals have properties that are not at all similar to elliptical galaxies, but are closer to irregular and late-type spiral galaxies, this term has been used to refer to all of the galaxies that share the properties of those above. These sorts of galaxies may in fact be the most common type of galaxies in the universe, but are much harder to see than other types of galaxies because they are so faint.
Because of the faintness of the lowest luminosity dwarf spheroidals and the nature of the stars contained within them, some astronomers suggest that dwarf spheroidals and globular clusters may not be clearly separate and distinct types of objects. Other recent studies, however, have found a distinction in that the total amount of mass inferred from the motions of stars in dwarf spheroidals is many times that which can be accounted for by the mass of the stars themselves. In the current predominantly accepted Cold Dark Matter cosmology, this is seen as a sure sign of dark matter, and the presence of dark matter is often cited as a reason to classify dwarf spheroidals as a different class of object from globular clusters (which show little to no signs of dark matter). Because of the extremely large amounts of dark matter in these objects, they may deserve the title "most dark matter-dominated galaxies" 
- Dwarf galaxy
- Dwarf elliptical galaxy
- Galaxy morphological classification
- Galaxy formation and evolution
- Groups and clusters of galaxies
- Irregular galaxy
- Local group
- List of nearest galaxies
- Dark galaxy
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- Simon, Josh; Geha, Marla (November 2007), "The Kinematics of the Ultra-faint Milky Way Satellites: Solving the Missing Satellite Problem", The Astrophysical Journal 670 (1): 313–331, Bibcode 2007ApJ...670..313S, DOI:10.1086/521816
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- Strigari, Louie; Koushiappas, et al; Bullock, James S.; Kaplinghat, Manoj; Simon, Joshua D.; Geha, Marla; Willman, Beth (September 2007), "The Most Dark Matter Dominated Galaxies: Predicted Gamma-ray Signals from the Faintest Milky Way Dwarfs", The Astrophysical Journal 678 (2): 614, arXiv:0709.1510, Bibcode 2008ApJ...678..614S, DOI:10.1086/529488