We also know that the Milky Way galaxy is not unique. In the 1920s, Edwin Hubble used the 100-inch optical telescope on Mount Wilson, in California, to find other galactic systems. In every direction, as far as the most powerful telescopes can see, there are galaxies of different sizes and shapes.
Galaxies can change.
Recent observations with optical, radio and X-ray telescopes show that deep in the central region or nucleus of many galaxies, some level of explosive activity is occurring. An extreme example of this activity is a quasar, where the nucleus can blaze forth at power levels a thousand times more than all the stars in a large galaxy. Most astrophysicists believe that this stupendous energy release is caused by matter falling into a supermassive black hole in the center of a galaxy.