There may be an invisible universe of stars, nebulas and galaxies made up entirely of dark matter. And astronomers now know how to look for it.
To put it simply, dark matter is a mystery. Astronomers have dozens of independent pieces of evidence that all point to the existence of some form of matter in the universe that is effectively invisible. It doesn't interact with light. But it does exert a gravitational influence on normal matter. Dark matter keeps galaxies glued together despite their high spin rates, keeps cluster gas cohesive despite its high temperature, bends the path of background light all over the universe, and even shapes the largest structures in the cosmos.
