When these hydrogen atoms are struck by highly energetic ultraviolet light, they are transformed from electrically neutral atoms to charged ions. The astronomers were surprised when they found far more hydrogen ions than could be explained with the known ultraviolet light in the universe, which comes primarily from quasars. The difference is a stunning 400 percent.
Strangely, this mismatch only appears in the nearby, relatively well-studied cosmos. When telescopes focus on galaxies billions of light years away—which shows astronomers what was happening when the universe was young—everything seems to add up. The fact that the accounting of light needed to ionize hydrogen works in the early universe but falls apart locally has scientists puzzled.
Another co-author of the story, CU-Boulder's Center for Astrophysics' Benjamin Oppenheimer, says that we don't know yet where this missing light is coming from:
If we count up the known sources of ultraviolet ionizing photons, we come up five times too short. We are missing 80 percent of the ionizing photons, and the question is where are they coming from? The most fascinating possibility is that an exotic new source, not quasars or galaxies, is responsible for the missing photons.
One of the possibilities is that this exotic matter is "the mysterious dark matter, which holds galaxies together but has never been seen directly." The light might be a product of this dark matter decaying over time.
source: gizmodo.com by Jesus Diaz