The vast majority of what is known about black holes comes from observations of AGN/quasars (super massive black holes that are "active"), which may be as luminous as ~0.5 quadrillion suns (1). This luminosity is powered by the quasar's central engine, which converts the gravitational potential energy of infalling matter to radiative energy, through means that are still debated. In an article in Nature commemorating the 50th anniversary of the discovery of quasars, a leading AGN researcher wrote "our best hope for understanding quasars is that extraterrestrials might drop in and explain them to us" (2). AGN/quasars are an underappreciated enigma.

(1) https://www.nature.com/articles/s41550-024-02195-x (2) https://www.nature.com/articles/495165a.pdf