Rupert Croft et al. 2016, "Large-Scale Clustering of Lyman Alpha Emission Intensity from SDSS/BOSS". arXiv:1504.04088, to appear in Monthly Notices of the Royal Astronomical Society Light is the principal messenger that astronomers have used to observe the Universe around us. By observing light, we learn that the Universe is made of galaxies, of which our one, the Milky Way, is one among many billions. Galaxies that are very faint on the sky, either because they are intrinsically emitting little light, or because they are very distant from us, require large optical telescopes to be observed. Our knowledge about the Universe has dramatically increased as bigger telescopes with more sensitive optical cameras to detect increasingly fainter galaxies have been developed. But faintness is not the only challenge that needs to be addressed. Just as important is to observe light that is increasingly diffuse. An astronomical object is more diffuse if its light is spread over a broader region of the sky. As an object becomes more diffuse, it is also harder to observe even if the total amount of light we receive from it stays fixed. Galaxies that are very diffuse might be undetectable to our telescopes even if the total light we receive from them is easily detectable when it comes from a point source, that is to say, a small source that emits all of its light from a small enough region to appear as a point in the sky. Therefore, the Universe might contain a lot of light that remains unknown to astronomy if it is very diffuse. The possible existence of very diffuse light is interesting because galaxies are known to contain a lot of dark matter, or matter that is invisible except for its gravitational effects on the ordinary matter that we can observe. The dark matter is broadly distributed around galaxies, forming structures that are called halos that surround each galaxy: the stars and gas clouds that emit light are concentrated in the central regions of a halo, whereas the dark matter is much more spread around. This is one of the reasons why researchers are interested in investigating the possible presence of any diffuse light which is distributed more similarly to the dark matter. The work by Rupert Croft and collaborators participating in the SDSS-III Collaboration, using data from the BOSS survey, attempts to search for this diffuse light around quasars in the distant Universe. These investigators have discovered the presence of an extremely diffuse light around quasars. Quasars are very luminous objects that are believed to be massive black holes that are accreting large amounts of matter. Because of their high luminosity, they can be used as markers of massive halos containing many galaxies, most of which are too faint to be observed in this survey, and then one can look for the presence of diffuse light around the quasars. The way the BOSS survey has been capable of detecting this light that is more diffuse than anything seen so far is not by using a big telescope, but by using a relatively small telescope that observes many objects at the same time over a broad field of view in the sky. Every hour, the BOSS survey took measurements of the light spectra of about 1000 objects, using optical fibers that are especially targeted. In total, more than a million objects were observed, among these nearly 200,000 quasars at high redshift. By collecting the observations made in fibers that are near a quasar on the sky, the investigators then checked if any of the light detected was coming from a diffuse halo around the quasar. They did this by using a special kind of light: it is called Lyman alpha light, which is emitted by hydrogen gas, the most abundant atom in the Universe. The Lyman alpha light has the advantage that is emitted at a special, known wavelength, which is then redshifted owing to the expansion of the Universe, just like the light we detect from galaxies or quasars. In this way, for every quasar we can search in any fibers placed in its proximity, and check if there is an excess of light that is present at the Lyman alpha wavelength at the redshift of the quasar. Usually each fiber is detecting light from many sources in the Universe: a lower redshift galaxy that was especially targeted, or other galaxies or stars along the line of sight that may add to the total light detected. These random objects vary randomly around every quasar. But by averaging over the observations made in hundreds of thousands of fibers that are close to any particular quasar, the investigators have found that there is a systematic excess of Lyman alpha light being emitted near the quasars. There are several possibilities to interpret this extremely diffuse light near quasars. One of them is that it originates in star-forming galaxies that follow a clustering pattern with the quasars. Any galaxies that actively form stars produce Lyman alpha light from glowing hydrogen gas that is heated near young, massive stars (an example of these glowing nebulae in our Galaxy is the Orion nebula). The known large-scale clustering measured in the Universe implies that these star-forming galaxies should be present in greater numbers near quasars than far from them, an effect induced by the large-scale structure that grows in the Universe due to the gravitational attraction of matter, called often the Cosmic Web. However, the level of diffuse Lyman alpha light detected is such that if this were the explanation, it would require a much larger amount of Lyman alpha photons to be produced and to escape from galaxies (without being absorbed by dust inside galaxies) than is expected. An alternative possibility is that the diffuse Lyman alpha light that has been discovered is caused by a physical effect from the quasars themselves. The quasars may be heating the surrounding intergalactic gas in a special way, causing an enhanced emission of Lyman alpha light. The difficulty here is that the diffuse emission needs to occur over an extremely large scale, up to a distance of millions of light-years from the quasars. To produce the observed amount of light, the quasars would need to produce an amount of energy that can heat intergalactic gas over this enormous volume around them that is as large as all the energy of the optical light that is observed to be emitted from them. One origin for this energy might be the hard ionizing radiation of quasars, produced at higher frequencies than the light we observe, which might ionize helium and be absorved over this volume by the intergalactic gas. Another possible origin is through the known jets produced by quasars, which are powerful streams of high-energy particles that are ejected by quasars at speeds close to the speed of light. These powerful jets might traverse distances of millions of light years and eventually deposit their energy to heat the intergalactic gas around quasars.