A mysterious, infrared light from the depths of space has been spotted by the Atacama Large Millimeter/submillimeter Array (ALMA). It's the faintest millimeter-wave source ever observed, and may tell researchers a bit more about the mysterious infrared background light filling the universe.

If you were to look into space, you'd see bright points of light where galaxies and stars were, and seemingly empty, dark spaces in between. But these spaces are anything but empty. Astronomers have found that there is a faint but uniform light, called the "cosmic background emission," coming from all directions. This emission consists of three main components called the Cosmic Optical Background (COB), the Cosmic Microwave Background (CMB) and the Cosmic Infrared Background (CIB).

Scientists have already figured out where the first two come from. The COB comes from a huge number of stars, and the CMB comes from hot gas just after the Big Bang. However, scientists have long wondered where CIB comes from.

"The origin of the CIB is a long-standing missing piece in the energy coming from the universe," Seiji Fujimoto, one of the researchers, said. "We devoted ourselves to analyzing the gigantic ALMA data in order to find the missing piece."

In this latest study, published in the Astrophysical Journal Supplement Series, the researchers tackled the mystery by examining data from the ALMA archive. ALMA has unprecedented sensitivity and resolution, which means it's a good tool to look for CIB.

In all, the researchers spent 900 days combing through data and looking for faint objects. They also looked for lensed sources, where gravity has magnified the source and has made even fainter objects visible to astronomers.

So what is the nature of these sources? The researchers found that 60 percent of them are galaxies which can also be seen in the optical/infrared images. Dust in galaxies actually absorbs optical and infrared light and re-emits the energy in longer millimeter waves which can then be detected by ALMA.

"However, we have no idea what the rest of them are," Masami Ouchi, one of the researchers, said. "I speculate that they are galaxies obscured by dust. Considering their darkness, they would be very low-mass galaxies. This means that such small galaxies contain great amounts of dust. That conflicts with our current understanding: small galaxies should contain small amounts of dust. Our results might indicate the existence of many unexpected objects in the distant universe. We are eager to unmask these enigmatic sources with future ALMA observations."