There are billions and billions of galaxies, composed of billions and billions of stars that shine light across the Universe. In between these galaxies are clouds of hydrogen and helium that indicate amounts of light. However, a study led by Juna Kollmeier of the Carnegie Institution For Science found that there is 400% more light in the Universe than can be explained by galaxies and quasars. The paper was published in The Astrophysical Journal Letters.

"It's as if you're in a big, brightly-lit room, but you look around and see only a few 40-watt lightbulbs," Kollmeier said in a press release. "Where is all that light coming from? It’s missing from our census.”

When ionizing ultraviolet photons from quasars pass through the intergalactic gas clouds, the neutral hydrogen atoms become charged, allowing scientists to measure them. Ionizing photons are also produced by really hot young stars, but those photons are retained within the galaxy and don’t affect intergalactic hydrogen. However, there aren’t enough quasars to produce the amount of light that has been measured in these intergalactic clouds.

However, this light-source deficit for hydrogen gas clouds only applies to galaxies that are relatively close to the Milky Way. The team used data from the Hubble Space Telescope’s Cosmic Origins Spectrograph to run simulations, and found that galaxies that are incredibly far away and are from early in the Universe’s history, the light source and gas measurements match up just fine. It isn’t clear why the age of the galaxies are causing this difference.

“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,” Kollmeier said. “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.”

Intergalactic hydrogen is fairly well-understood, so the researchers believe the problem is more likely to stem from an unknown light source rather than an error within the simulation itself.

“The most exciting possibility is that the missing photons are coming from some exotic new source, not galaxies or quasars at all,” added co-author Neal Katz. "You know it's a crisis when you start seriously talking about decaying dark matter!"

Dark matter is one of the biggest mysteries of science, and it is believed to make up about 95% of all matter in the Universe. Though dark matter is still theoretical and has never been observed, the team hypothesized that it could generate the ionizing photons needed to light up the hydrogen as it decays. It is possible that there is simpler explanation for this phenomenon, and researchers will continue to search for the source of this inconsistency.

“The great thing about a 400% discrepancy is that you know something is really wrong,” stated co-author David Weinberg. “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.”