Early galaxies formed stars much more quickly than today's galaxies. To form lots of stars, a galaxy must have large reservoirs of gas. Astronomers theorized that these young galaxies were being fed by streams of primordial gas leftover from the Big Bang that could be used to form stars. Unfortunately, this theory is hard to test because the gas is so pure and thinly spread that it doesn't emit much light.
Lucky for us, one such star forming galaxy (Q1442-MD50) is aligned with a very bright galaxy behind it. When light from the background galaxy passes through the streams of gas feeding the star forming galaxy, some of that light gets absorbed by the gas in the stream. Astronomers were able to observe this absorption, confirming that these young, star-forming galaxies are indeed fed by filaments of gas flowing inward. In addition, the particular light that gets absorbed tells us what makes up the streams. The gas surrounding Q1442-MD50 was found to contain deuterium, an atom that was created during the Big Bang along with hydrogen and helium. Deuterium is very easily destroyed, so its presence in these gas streams tells us that this gas has not interacted with any environment that contains stars; it is indeed pristine gas leftover from the Big Bang.
Astronomers intend to search for more galaxies with these chance alignments, helping us further our knowledge about how galaxies form and evolve.