The center of the Milky Way is believed to contain a supermassive black hole, with a mass of about one million Suns. As the adjective suggests, it is the largest black hole in the galaxy; and it is believed that all galaxies have one at their centers.
The recognition of that structure started with the radio astronomy observations of Karl Jansky, who found that there were strong radio emissions from the nucleon of the Milky Way, produced by high speed electrons circling magnetic fields, which indicated the existence of a compact central object, the source of these electrons. Because visible light from the center of the Milky Way is absorbed by interstellar dust before it reaches the Earth, the composition of the center must be determined by infrared and radio observations. From these, it appears that material in the innermost spiral arm around the center of the Milky Way is expanding away from the center. At the very center, astronomers have identified a source of radio emissions of enormously high energy, but no larger than the solar system — it is presumed to be a supermassive black hole.
Supermassive black hole (Wikipedia)