Black holes have long served as fodder for science fiction — and for good reason. These unimaginably dense objects contain so much matter trapped in such a small volume that their gravity is strong enough to prevent even light from escaping their surfaces.

Although the first prediction of a black hole was made nearly 250 years ago by the English philosopher and clergyman John Michell, the first black hole candidate, Cygnus X-1, wasn't discovered until 1971. Since then, astronomers have tirelessly chipped away at countless questions related to these once-mythical beasts. But one of the most basic and enduring questions remains: Do they come in all sizes?

Small and large, or small to large?

Over the past few decades, astronomers have compiled loads of evidence for the existence of black holes at both ends of the mass spectrum. Researchers have uncovered small black holes that weigh just a few to 100 times the mass of the Sun, as well as supermassive black holes that can reach billions of times the mass of their star-sized brethren.

Stellar-mass black holes are thought to form when a relatively massive star dies in spectacular fashion. As the exhausted star burns through its final traces of fuel, its immense gravity causes it to collapse in on itself. If the collapsing star isn't too big, the infalling material rebounds off the star's dense core. This causes a supernova explosion, often leaving behind a tiny white dwarf or neutron star. But if the surviving remnant is greater than about three solar masses, not even tightly packed neutrons can prevent the city-sized core from continuing to collapse into a stellar-mass black hole.

On the other hand, there also exists a class of black holes known as supermassive black holes, which serve as the central gravitational anchors of most, if not all, large galaxies. Though supermassive black holes are anywhere from millions to billions of times the mass of the Sun, they pack all that matter into a region roughly the size of a single star. There are many lines of evidence that indicate these cosmic behemoths are common throughout the universe, but exactly how and when they formed still remains a mystery.