Yes, it is currently believed that most, if not all, galaxies contain supermassive black holes at their cores. These supermassive black holes, with masses millions or even billions of times that of our Sun, are commonly referred to as active galactic nuclei (AGN) when they are actively accreting matter and emitting intense radiation.
While it may seem that our supermassive black hole, known as Sagittarius A* (Sgr A*), is the only one visible from Earth, that is not entirely accurate. In fact, astronomers have observed and detected numerous AGNs in other galaxies throughout the universe. The reason Sgr A* is often mentioned in discussions is that it is the supermassive black hole closest to us, located at the center of our Milky Way galaxy, making it more accessible for observation and study.
The primary reason that AGNs in other galaxies appear differently from Earth compared to Sgr A* is due to distance and orientation. Galaxies are vast systems located at significant distances from us, often millions or billions of light-years away. AGNs within these galaxies can vary in their activity levels, depending on factors such as the amount of accreting material and the orientation of the black hole's relativistic jets relative to our line of sight.
When an AGN is highly active and emitting substantial amounts of radiation, it is classified as a quasar. Quasars can be visible from great distances because their intense emission makes them among the brightest objects in the universe. However, not all AGNs are active quasars at all times. Some may be less luminous or obscured by dust and gas in their host galaxy, making them harder to observe.
Furthermore, the light from distant AGNs has to travel vast distances through space before reaching Earth. This journey can result in the absorption and scattering of light by intervening material, making AGNs fainter or harder to detect.
Overall, while our Milky Way's supermassive black hole, Sgr A*, is the most studied and visible from Earth due to its proximity, astronomers have observed and continue to study numerous AGNs in other galaxies, including quasars, which provide valuable insights into the role of supermassive black holes in galaxy formation and evolution.