https://www.scientificamerican.com/article/glittering-diamond-dust-in-space-might-solve-a-20-year-old-mystery/

When astronomers first peered at the cosmos in microwave light, they knew they had stumbled on a window into the universe’s earliest moments. After all, the cosmic microwave background—that hazy afterglow of the big bang released when the universe was a mere 380,000 years old—has allowed scientists to answer fundamental questions about where we came from. But microwave light has also raised an intriguing mystery closer to home. In 1996 astronomers noticed an inexplicable excess of microwaves emanating from our own galaxy. For over 20 years, this so-called anomalous microwave emission has remained an enigma—until today. A new study published in Nature Astronomy suggests spinning nano-diamonds might be the culprit.

Ten years ago, while studying nascent planetary systems forming in whirling disks of gas and dust around young stars, Cardiff University astronomer Jane Greaves noticed a few of those systems seemed to be faintly glowing with microwaves. She initially attributed the glow to flaws in her data but later reconsidered after hearing a colleague’s talk about anomalous microwave emission. Returning to the telescope, she and her collaborators monitored 14 young star systems for mysterious microwave emissions, finally finding three radiating that telltale glow. Those same three systems, it turns out, are also the only three within Greaves’s sample known to host nano-diamonds—pint-size, pyramid-shaped crystals containing only hundreds of carbon atoms, all sheened with an atoms-thin gloss of frozen hydrogen likely accumulated from the interstellar medium. “This really is a clue of nature telling us nano-diamonds are what is responsible” for the anomalous microwaves, she says.