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As we know, a galaxy is a huge collection of millions, billions, or trillions of stars. The galaxy we live in is called the Milky Way. The name comes from the literal translation of the Latin "Via Galactica", with via meaning "road"or "way"and the word galaxy from the Greek root "gala", meaning "milk". The Milky Way galaxy is composed of about 400 billion stars and is about 100,000 light years in diameter.The Milky Way is classified as a spiral galaxy and is composed of three main regions: 1. the disk, where the solar system is located, 2. the central bulge at the core, which is densely packed with stars and presumed to have a black hole at its center, and 3. a halo,a diffuse region with a low density of stars, that surrounds everything. The halo is believed to be composed mainly of dark matter that exceeds even beyond its ends. The total mass of the Milky Way is assumed to be at least 600 billion times the mass of the sun, while the densely packed visible part is only 200 billion times the mass of the sun. This discrepancy in numbers is believed to be caused by the dark matter in the halo, since it seems to be taking up mass, but doesn't emit or radiate light. This "missing mass"accounts for almost 90% of the mass in the universe. Even though scientists don't know what it is, they know it's there because they can detect it by the gravitational effect it has on the surrounding visible objects.Unfortunately, this unknown dark matter is also the determining factor in the evolutionary future of the universe. If there is too little of it to gravitationally bind the the universe together, it can continue expanding forever. If there is enough, though, the universe might slow down the expansion, come to a halt, and begin to contract and eventually collapse. This is why it is so important to find out just what dark matter is and how much of it there is.Alina Vayntrub -- 2000