Well, a discovery the Hubble Telescope was responsible for is dark matter, which has a unique composition compared to gas and dust in galaxies, and makes up most of the universe. A dark matter ring was discovered in 2007 at the site of two galaxy clusters’ collision, 5 billion light-years away from Earth. It was detected by the deflection, or bending of light in background galaxies when looking at the two galaxies’ collisions. The dark matter is probably a result of the high gravity following the collision. Evidence for dark matter, therefore, is indirect, only detectable when galaxies in the foreground caused distortions in the light of galaxies in the background.
Currently, the Hubble Space Telescope’s ACS is being used to survey dwarf galaxies 250 million light years away, in the Perseus Cluster. It is still too far away to detect small stars that may be too faint, but larger, more luminous stars are still readily detected. The Hubble Space Telescope revealed that at least 12 of the stars in the Perseus Cluster require dark matter. The stars appear smoother, scientists believe, because of the presence of dark matter, relative to the stars in spiral galaxies such as the Milky Way. The dark matter would protect these dwarf galaxies, whose stars have a high mass to light ratio (Hubble made it possible to calculate their mass by measuring the amount of X-ray radiation emitted by the hot gases of the stars), from being destroyed by outside gravitational pull. The discovery of this phenomenon would have been impossible without the incredibly high detail provided by a space telescope such as Hubble. As previously mentioned, ground telescopes have too much background light interference and not enough resolution for study of individual stars in galaxies outside our own.  
1. Hubble Finds Ring of Dark Matter. 5 Oct 2010. Available:
2. Hubble Provides New Evidence for Dark Matter in Small Galaxies. 14 Nov 2010. Available:
Edit: If there's anything I want you to get from these recent posts on the Hubble, it's that funding for space programs helps us understand so many new things about the universe we live in and its beautiful physical laws. Dark matter would have probably remained a mystery if it had not been for the images from Hubble. Please write to your representatives urging them to continue supporting NASA or PBS, which has fascinating programs to educate us all.