Galaxy , any of the systems of stars and interstellar matter that make up the universe. Many such assemblages are so enormous that they contain hundreds of billions of stars. Galaxies are sprawling systems of dust, gas, dark matter, and anywhere from a million to a trillion stars that are held together by gravity. Nearly all large galaxies are thought to also... In modern times, though, we use “ galaxy ” more generically to refer to any large system of stars, gas, dust and dark matter, all held together by their mutual gravity. A galaxy can contain hundreds of billions of stars and be many thousands of light-years across. Our own galaxy , the Milky Way, is around 100,000 light-years in diameter.
