The galaxies of the Universe are afloat in a sea of stars, according to a paper released by Science. These once inhabited galaxies but were pulled out of their former homes by gravitational interactions during galaxy collisions and near misses.

These conclusions come from an exploration of something called the "extragalactic background light," a diffuse infrared glow that was first identified using data from the Spitzer space telescope. There are plenty of sources of infrared light in the Universe that we know of but it's possible to account for them—large surveys have told us where stars and galaxies are. After those were accounted for, there was still a general glow in the infrared wavelengths that Spitzer imaged.

A dull infrared glow may not sound interesting, but people came up with some intriguing potential explanations for its existence. Two of them suggest that the light originates early in the Universe's history, during a period called "reionization," when light from the first stars and galaxies started to strip electrons from the Universe's hydrogen. By now, even energetic light from that era has been red-shifted so much that it resides in the infrared.

The diffuse light, then, could come from the earliest of galaxies. Alternatively, some scientists have suggested that the light comes from the predecessors of the supermassive black holes that now inhabit the centers of galaxies. It has been proposed that these black holes aren't the result of the death of a star; instead, they may have formed by the direct collapse of a massive gas cloud.

To understand the infrared background, you have to do your observations above the atmosphere, which has many molecules that absorb infrared radiation. Since it's expensive to put the hardware into orbit, an international team put together CIBER, the Cosmic Infrared Background Experiment. It's a pair of small telescopes, able to be lofted above the atmosphere by a small sounding rocket. CIBER's been launched twice, done some imaging, and now some of the people behind it are attempting to interpret what it observed.

They start by taking their full data and eliminating other sources of infrared light. Light from warm dust in our own Solar System was handled by the two trips, which took place at different times of the year. Since this light should be influenced by Earth's position within the Solar System, this handles the local dust. Our galaxy's dust has been measured by other instruments. And surveys of stars and galaxies performed by other telescopes can account for over 99.9 percent of the light from stars.

With all that accounted for, there was still some infrared light—the extragalactic background light. The authors then considered three possible explanations for it. If it was produced by some of the Universe's first galaxies, then there'd be a lot of those galaxies. As a result, we'd expect to see a lot more of the heavier elements produced by these galaxies, along with an x-ray background; given this discrepancy, the authors consider this explanation "difficult to justify." The direct collapse of black holes doesn't work well to explain the color of the background or its intensity at specific wavelengths, so that doesn't work either.

What they seem to be left with is stars—lots of them. These stars would have been pulled out of the known galaxies and either orbit far out into the halo or wander intergalactic space. Given that the light in the background is roughly similar to the amount produced by all of the light produced by the galaxies we've identified, the results suggest that galaxies may hold only half the stars in the Universe.

That conclusion, however, should be viewed as pretty tentative. The work depends on a number of attempts to quantify known sources of infrared light, so there's an opportunity for people to pick holes in it there. Theorists may also quibble with the properties assigned to the early galaxies and black holes. And there's always the possibility that an explanation for the background that the authors hadn't considered will be proposed. Still, if the results hold up to scrutiny by the research community, it will mean that the chaotic interactions of the earliest galaxies have left the Universe swimming in stars.

Science, 2014. DOI: 10.1126/science.1258168 (About DOIs).