KISSIMMEE, FLORIDA—To understand the universe’s very first galaxies, astronomers need to know what they’re made of. But the clouds of gas that birthed them don’t shine with stars like a fully formed galaxy, making them very hard to see. Now, a team has developed a method for measuring the size and mass of clouds of cool, dense gas that existed 11 billion years ago, when the universe was less than 20% of its current age. In the process, they have confirmed that they are big enough and massive enough to form galaxies. Astronomers had previously been able to detect the presence of such clouds if there was a bright source of light behind them like a quasar—the energetic black hole at the center of a galaxy. As the quasar’s light passes through the gas cloud, hydrogen atoms absorb a specific wavelength of light through a process called Lyman-α absorption. That absorption line in the quasar’s spectrum is a signature of such clouds, which are consequently known as damped Lyman-α (DLα) systems. But the thin beams from quasars don’t reveal whether the DLA cloud is just a small clump or galaxy-sized. Today, a pair of astronomers told the American Astronomical Society meeting here that they have instead used the light from entire galaxies to probe DLα clouds—the equivalent of using searchlight beams compared with a quasar’s laser. With such a broad backlight, they are able to say that the clouds are at least as big as typical galaxies from that era. The image above shows a quasar (white, right) shining its narrow beam through a DLα cloud (center) to Earth; superimposed on the quasar is a galaxy (red) shining a much wider beam through the cloud. To date, the researchers have studied more than 10 such clouds measuring up to 30,000 light-years across. From details of the absorption they can estimate the clouds’ densities, and from that their masses. The technique requires many hours of observations with the world’s largest telescopes, which makes it hard to apply widely now. But when the next generation of 30-meter-class telescopes comes online in the next decade, the astronomers say they will be able to routinely probe the cradle of early galaxies.