Could Supermassive Black Holes Explain Our Universe's Gravitational-Wave 'Hum'?

"Earlier this year, after 15 years of searching, scientists finally heard the background hum of low-frequency gravitational waves that fill our universe," writes Space.com.
"Now, the hard work of searching for the source of these ripples in spacetime can begin."
Currently, the primary suspects in this case are pairings of supermassive black holes with masses millions, or even billions, of times that of the sun. However, that doesn't mean that there isn't room for a few unusual suspects, which co