"The most exciting possibility is that the missing photons are coming from some exotic new source, not galaxies or quasars at all," said Neal Katz of the University of Massachusetts at Amherst. For example, the mysterious dark matter, which holds galaxies together but has never been seen directly, could itself decay and ultimately be responsible for this extra light. You know it's a crisis when you start seriously talking about decaying dark matter!"
The vast reaches of empty space between galaxies are bridged by tendrils of hydrogen and helium, which can be used as a precise "light meter." In a recent study published in The Astrophysical Journal Letters, a team of scientists finds that the light from known populations of galaxies and quasars is not nearly enough to explain observations of intergalactic hydrogen. The difference is a stunning 400 percent.
"The great thing about a 400% discrepancy is that you know something is really wrong," commented co-author David Weinberg of The Ohio State University. "We still don't know for sure what it is, but at least one thing we thought we knew about the present day universe isn't true."
"It's as if you're in a big, brightly-lit room, but you look around and see only a few 40-watt light bulbs," noted Carnegie's Juna Kollmeier, lead author of the study. "Where is all that light coming from? It's missing from our census."
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