Final answer:
The center of our Galaxy, the Milky Way, contains a supermassive black hole known as Sagittarius A*. This has been determined through observations of high-speed stellar orbits, radio, and X-ray emissions surrounding the galactic center, which suggest a mass that only a black hole could explain. The galaxy also contains interstellar gas and dust, as well as mysterious dark matter making up most of its mass.
Step-by-step explanation:
At the center of our Galaxy, known as the Milky Way, lies an astronomical feature known as Sagittarius A* (Sgr A*). Sgr A* is a supermassive black hole, which is indicated by the very large amount of mass crammed into an extremely small volume observed at the galactic center. Because black holes emit no radiation, they are identified through the gravitational effects they have on surrounding matter, such as stars. These stars orbit at a high speed due to the massive gravitational pull from a dense core, which is consistent with the presence of a black hole and not a dense cluster of stars or other ordinary matter.
Radio and X-ray observations have detected energetic phenomena at the galactic center which further supports the existence of a black hole. This discovery is a result of cumulative efforts by scientists in various fields over many decades using advanced instruments capable of probing the physical universe.
Aside from the black hole, the Milky Way is also characterized by vast quantities of interstellar gas and dust, serving as the raw material for new star formation, and the galaxy's evolution. Furthermore, there is the intriguing and predominant presence of dark matter, a substance that composes a significant portion of the total mass of the galaxy yet remains undetected by conventional observational techniques.