Earlier this year, astronomers got their first good look at the intergalactic medium (IGM), the “dim matter” that links galaxies together into a single cosmic web. Last week, a team of researchers led by Carnegie astronomer Juna Kollmeier examined the composition of ionized dim matter connecting nearby galaxies, and found something very strange: amounts of visible hydrogen and helium suggesting the presence of four times as many photons as the nearby galaxies could plausibly have produced. As Kollmeier puts it:

“It’s as if you’re in a big, brightly-lit room, but you look around and see only a few 40-watt lightbulbs. Where is all that light coming from? … Either our accounting of the light from galaxies and quasars is very far off, or there’s some other major source of ionizing photons that we’ve never recognized. We are calling this missing light the photon underproduction crisis. But it’s the astronomers who are in crisis — somehow or other, the universe is getting along just fine.”

And if you’re thinking this might not just be an error on the part of astronomers, co-author Neal Katz has the same thought: “The most exciting possibility is that the missing photons are coming from exotic new source, not galaxies or quasars at all.” The most orthodox working theory is that the extra light is produced by dark matter decay, but since nobody knows what dark matter is (much less how it decays), that’s not a very persuasive explanation. “You know it’s a crisis,” said Katz, “when you start seriously talking about decaying dark matter!” It’s worth noting that the leading dark matter candidate, sterile neutrinos, would theoretically produce photons on decay—but so would almost anything else, so that’s not necessarily much help.

Wherever this light came from, its effects are visible only in nearby galaxies—which means that either the light is local to us, or (more likely) that it reflects a process that happened relatively recently in the cosmic story, after the light we’ve received from faraway galaxies was transmitted. Remember that nothing we’re looking at in space is current—when we look up and see the north star Polaris 434 light years away, that means we’re looking at Polaris as it was 434 years ago, in 1580. Polaris could be gone now, or ten times as large, or bright purple, and we’d be none the wiser. So the fact that the effects of this light are visible only in relatively nearby galaxies only tells us that they weren’t a property of the early universe.

What do you think the light is? Share your thoughts in the comments below.