Galaxies and exotic Objects
Galaxies are aggregations of many hundreds of billions of stars, along with gravitationally bound clouds of gas and dust and nearby star clusters. As big as they are, they can be regarded as a sort of basic building block of the universe - the majority of the visible mass in the known universe is concentrated in galaxies. Like stars, galaxies are always evolving, and different kinds of galaxies are thought to be in different stages of their evolution. As we observe galaxies farther away in space (and consequently in time), we see more and more unusual objects, like quasars. It is now thought that most galaxies may contain huge black holes at their centers.