Dark matter is a form of matter thought to account for approximately 85% of the matter in the universe and about a quarter of its total energy density. Most dark matter is thought to be non-baryonic in nature, possibly being composed of some as-yet undiscovered subatomic particles. [a] Its presence is implied in a variety of astrophysical observations, including gravitational effects which cannot be explained by accepted theories of gravity unless more matter is present than can be seen. For this reason, most experts[who?] think dark matter to be abundant in the universe and to have had a strong influence on its structure and evolution. Dark matter is called dark because it does not appear to interact with observable electromagnetic radiation, such as light, and is thus invisible to the entire electromagnetic spectrum, making it undetectable using existing astronomical instruments.
The original goal of Cassini-Huygens was to study Saturn and its large, misty, tortured, moon Titan. Titan, the second-largest moon in our Solar System, after Ganymede of Jupiter, is a world long-shrouded in mystery, hiding behind a thick orange veil, and slashed with hydrocarbon lakes and seas. However, there are other enticing moons known to circle the ringed planet. Saturn's mid-sized icy moons (Mimas, Enceladus, Tethys, Dione, Rhea, Hyperion, Iapetus, and Phoebe) are enchanting worlds. Each one of these frozen little moons reveals an interesting and unique geology. So far, Saturn is known to sport 62 icy moons!
Titan has three large seas. However, the seas of Titan are not filled with water, but are filled instead with swirling liquid hydrocarbons. All three of Titan's exotic seas are close to its north pole, and they are surrounded by many smaller hydrocarbon-filled lakes in the northern hemisphere.
Earth's Moon is enchanting; bewitching. The face of the "man"--that some cultures see etched on its brilliant surface--is really composed of the dark areas of the lunar maria (Latin for "seas"), and the lighter highlands of the Moon's surface. Some cultures tell of other examples of strange images seen on the Moon's lovely disk, such as the "Moon Rabbit".