Quasars are rare cosmic objects in the universe, which is why astronomers are struggling to explain the discovery of a cosmic "quartet" of quasars in close physical proximity to each other.

Quasars — created by the coming together of matter at the heart of a galaxy into a supermassive black hole — exist only during a brief phase in the evolution of a galaxy.

While they are the most luminous objects in the universe, outshining their host galaxy's entire population of stars, they only exist in that hyper-luminous form for a tiny fraction of a galaxy's lifetime, dying down as the inflow of matter slows down, astronomers say.

That makes them rare, and even when detected, they're generally found separated from each other by hundreds of millions of light years.

Only around 100 pairs of quasars have been found, and only two sets of triplets have been detected.

That makes the discovery of the quasar foursome in a giant "proto-cluster" of galaxies surrounded by a vast nebula of cool, dense gas, totally unexpected, the researchers say.

The odds of seeing such an arrangement of quasars are something on the order of 10 million to one, they say in their study published in the journal Science.

"The discovery is significant both because there are four of them, and because they are so close together," says lead researcher Joseph Hennawi of the Max Planck Institute for Astronomy in Germany.

The observation of a giant cloud of cool gas in the proto-cluster of galaxies suggests quasars can form in more different environments than are predicted by current models, he says.

"Current models of how structure forms in the universe would never predict that there would be so much cool, dense gas around," he explains. "Instead, those models predict that the gas in such a massive object should be 1,000 times hotter and 1,000 times less dense."

The existence of the quasar quartet in an environment not thought conducive to their creation means there is likely some physical process at work that current models aren't capturing, he says.

"If you discover something which, according to current scientific wisdom, should be extremely improbable, you can come to one of two conclusions: either you just got very lucky, or you need to modify your theory," Hennawi says.

The quasar quartet was discovered during a sky survey conducted at the W.M. Keck Observatory in Hawaii.