Galaxy protoclusters, which will eventually grow into the massive clusters we see in the local Universe, are usually traced by locating overdensities of galaxies¹. Large spectroscopic surveys of distant galaxies now exist, but their sensitivity depends mainly on a galaxy’s star-formation activity and dust content rather than its mass. Tracers of massive protoclusters that do not rely on their galaxy constituents are therefore needed. Here we report observations of Lyman-α absorption in the spectra of a dense grid of background galaxies^2,3, which we use to locate a substantial number of candidate protoclusters at redshifts 2.2 to 2.8 through their intergalactic gas. We find that the structures producing the most absorption, most of which were previously unknown, contain surprisingly few galaxies compared with the dark-matter content of their analogues in cosmological simulations^4,5. Nearly all of the structures are expected to be protoclusters, and we infer that half of their expected galaxy members are missing from our survey because they are unusually dim at rest-frame ultraviolet wavelengths. We attribute this to an unexpectedly strong and early influence of the protocluster environment^6,7 on the evolution of these galaxies that reduced their star formation or increased their dust content.

星系原星团最终将成长为我们在本地宇宙中看到的巨大星团，通常通过定位超密度星系¹来追踪。现在存在对遥远星系的大型光谱调查，但它们的灵敏度主要取决于星系的恒星形成活动和尘埃含量，而不是其质量。因此，需要不依赖于其星系成分的大型原星团的示踪剂。^{在这里，我们报告了在背景星系密集网格2,3}的光谱中观察到 Lyman-α 吸收，我们用它来定位大量候选原星团，通过它们的星系际气体在红移 2.2 到 2.8 之间。^{我们发现，与宇宙学模拟4,5}中类似物的暗物质含量相比，产生最多吸收的结构（其中大部分是以前未知的）包含的星系令人惊讶地少。几乎所有的结构都被认为是原星系团，我们推断他们的一半预期星系成员在我们的调查中丢失了，因为它们在静止帧紫外波长下异常暗淡。我们将此归因于原星团环境^6,7对这些星系演化的意外强烈和早期影响，这些影响减少了它们的恒星形成或增加了它们的尘埃含量。