The structure of the universe is often described as being a cosmic web of filaments, nodes, and voids, with the nodes being clusters of galaxies, the largest gravitationally bound objects known. These nodes are thought to have been seeded by small-amplitude density fluctuations like those observed in the cosmic microwave background (CMB) which grew until they collapsed into the structures seen today. While the CMB is well understood, and the details of present-day galaxy clusters are well-described, the intermediate phases of evolution lack sufficient observations to constrain the models. Traditional galaxy cluster searches assume these objects have had enough time to equilibrate so that the intergalactic gas has heated up enough to be detected in X-ray emission. To detect the more distant galaxies and protoclusters that are too faint to detect in the X-ray, astronomers use their bright infrared or submillimeter emission instead.
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Source: Phys.org