One of the basic building block of the universe, a galaxy is a massive system of stars, stellar remnants, gas, dust, and possibly a hypothetical substance known as dark matter, bound together by gravity. Galaxies may be anywhere from 1 to 100,000 light years across and are typically separated by millions of light years of intergalactic space. They are grouped into clusters, which in turn can form larger groups called superclusters and sheets or filaments. There are many different kinds of galaxy including spiral (like our own Milky Way galaxy), elliptical, ring, dwarf, lenticular and irregular. There are estimated to be over a hundred billion galaxies in the observable universe.