冷暗物质（或CDM）是大爆炸理论在改善的过程中加入的新材料，这种物质在宇宙中不能用电磁辐射来观测，因此是暗的；同时这种微粒的移动是缓慢的，因此是冷的。在2006年，多数的宇宙学家热衷于描述冷暗物质如何在早期宇宙仍是平滑的初始状态下（如宇宙微波背景辐射所示），如何形成如同我们今日所见的星系和星系团的结构－宇宙的大尺度结构。

在冷暗物质的理论中，结构依层级增长，在连续和逐级增长的过程中，少量的物质先塌缩和合并在一起，逐渐形成越来越巨大的结构。在流行在80年代的热暗物质的事例中，结构不依层级增长（由下而上），而是以断裂的方式发展（由上而下），最巨大的超星系团先形成像船舱甲板似的，像薄煎饼的层状结构，然后断裂形成，像星系－我们的银河系－这样的小碎片。热暗物质预测的与观察到的大尺度结构大相迳庭，而冷暗物质的事例却总能与观测的现象契合。

In cosmology and physics, cold dark matter (CDM) is a hypothetical form of matter (a kind of dark matter) whose particles moved slowly compared to the speed of light (the cold in CDM) since the universe was approximately one year old (a time when the cosmic particle horizon contained the mass of one typical galaxy); and interact very weakly with ordinary matter and electromagnetic radiation (the dark in CDM). It is believed that approximately 84.54% of matter in the Universe is dark matter, with only a small fraction being the ordinary (baryonic) matter that composes stars, planets and living organisms. Since the late 1980s or 1990s, most cosmologists favor the cold dark matter theory (specifically the modern Lambda-CDM model) as a description of how the Universe went from a smooth initial state at early times (as shown by the cosmic microwave background radiation) to the lumpy distribution of galaxies and their clusters we see today — the large-scale structure of the Universe. The theory sees the role that dwarf galaxies played as crucial, as they are thought to be natural building blocks that form larger structures, created by small-scale density fluctuations in the early Universe.