Read: What keeps black holes from expanding everywhere?

A pair-instability supernova happens when the core grows so hot that light begins to spontaneously convert into electron-positron pairs. The light’s radiation pressure had kept the star’s core intact; when the light transforms into matter, the resulting pressure drop causes the core to rapidly shrink and become even hotter, further accelerating pair production and causing a runaway effect. Eventually the core gets so hot that oxygen ignites. This fully reverses the core’s implosion, so that it explodes instead. For cores with a mass from about 65 to 130 times that of our sun (according to current estimates), the star is completely obliterated. Cores from about 50 to 65 solar masses pulsate, shedding mass in a series of explosions until they drop below the range where pair instability occurs. Thus, there should be no black holes with masses in the 50-to-130-solar-mass range.

“The prediction comes from straightforward calculations,” said Woosley, whose 2002 study of this “pair-instability mass gap” is considered definitive.

Black holes can exist on the other side of the mass gap, weighing in at more than 130 solar masses, because the runaway implosion of such heavy stellar cores can’t be stopped, even by oxygen fusion; instead, they continue to collapse and form black holes. But because stars shed mass throughout their life, a star would need to be born weighing at least 300 suns to end up as a 130-solar-mass core, and such behemoths are rare. For this reason, most experts assumed that black holes detected by LIGO and Virgo should top out at about 50 solar masses, the lower end of the mass gap. (The million- and billion-solar-mass supermassive black holes that anchor galaxies’ centers formed differently, and rather mysteriously, in the early universe. LIGO and Virgo are not mechanically capable of detecting the collisions of supermassive black holes.)

That said, a few experts did boldly predict that black holes in the mass gap would be seen—hence the 2017 bet.

At a meeting that February at the Aspen Center for Physics, Belczynski and Daniel Holz of the University of Chicago wagered that “black holes should not exist in the mass range between 55 and 130 solar masses, because of pair instability,” and thus that none would be detected among LIGO/Virgo’s first 100 signals. Woosley later co-signed with Belczynski and Holz.

But Carl Rodriguez of the Massachusetts Institute of Technology and Sourav Chatterjee of the Tata Institute of Fundamental Research in Mumbai, India, later joined by Fred Rasio of Northwestern University, bet against them, wagering that a black hole would indeed be detected in the mass gap, because there’s a roundabout way for these plus-size black holes to form.

Whereas most of the colliding black holes that wiggle LIGO’s and Virgo’s instruments probably originated as pairs of isolated stars (binary star systems being common in the cosmos), Rodriguez and his co-signers argued that a fraction of the detected collisions occur in dense stellar environments such as globular clusters. The black holes swing around in one another’s gravity, and sometimes they catch one another and merge, like big fish swallowing smaller ones in a pond.