Black holes vary in size. Some astrophysicists have theorized black holes may be as small as a mountain while others may be over twice the size of our own galactic core. It is theorized small, invisible' black holes are orbiting our own galactic core the same way solar systems do. Because black holes do not generate light, small black holes are very difficult to detect. Unless interacting with a star, a small black hole is essentially invisible.
Supermassive' black holes are estimated to range from millions to billions of solar masses. 'Stellar' black holes, created by the gravitational collapse of massive stars, are much smaller, range from 3 to a 1000 solar masses, and are much more common. Supermassive black holes typically act as galactic cores, with the smaller black holes maintaining an orbit around them. (There is no reason to believe small black holes cannot grow' by collecting more and more matter over time.)
Each galaxy contains a galactic core at its center, typically a 'supermassive' black hole. Supermassive black holes, or galactic cores, have a surface gravity so strong no light can escape from it. Light from the surrounding regions can escape, but may suffer from gravitational redshifting in the process. It is assumed any form of matter entering a black hole will never again emerge. For several decades, this lack of light from the center of our galaxy, and others, led most astrophysicists to believe the stars and solar systems orbited a hollow, empty space.
Galactic cores display field characteristics. They generate an incredibly powerful gravity field. They have magnetic north and south poles, indicated by the massive jets of electrons shooting outward from each pole. (It is generally assumed positrons are not expelled.)
The stars and solar systems around galactic cores tend to move into orbits perpendicular to the magnetic poles, or around the core's equator. The size of galactic cores and their surrounding galaxies are proportional. The larger the galactic core, the larger its surrounding galaxy. The orbital speed of a galaxy's outer stars is determined by the size of its galactic core. Combining the observations of poles in galactic cores with the evidence of a massive gravity field leads to the conclusion of a galactic core's energy fields being both magnetic and gravitational in nature.
Recently, it was theorized supermassive black holes gradually evolve into quasars by absorbing other galaxies, including their supermassive black holes.