The Cosmic Microwave Background looks mostly smooth, but tiny hot and cold spots exist. What fundamental structures in the universe started forming from these tiny spots?

The Cosmic Microwave Background (CMB) is the faint afterglow radiation from the Big Bang, representing a snapshot of the universe when it was only about 380,000 years old. Before this time, the universe was a hot, dense plasma where photons (light particles) were constantly scattering off free electrons and protons, preventing light from traveling freely. As the universe expanded and cooled, electrons combined with protons to form neutral hydrogen atoms, a process called recombination. This made the universe transparent, allowing photons to travel unimpeded, and these photons are what we observe today as the CMB. While the CMB appears largely uniform, tiny variations in its temperature, known as anisotropies or hot and cold spots, exist. These minuscule temperature differences, typically less than one part in 100,000, correspond to equally tiny variations in the density of matter in the early universe. The slightly colder spots represent regions of slightly higher density, and the slightly hotter spots represent regions of slightly lower density. These initial density fluctuations were the fundamental seeds from which all the large-scale structures we observe in the universe today began to form. Gravity, the attractive force between objects with mass, is the key mechanism. Regions with a slightly higher density had a slightly stronger gravitational pull than their surroundings. Over vast stretches of cosmic time, gravity amplified these density differences. The primary structures that started forming from these tiny spots were the first galaxies and, subsequently, clusters of galaxies. Dark matter played a crucial role. Dark matter is a mysterious form of matter that does not interact with light or other electromagnetic radiation, but it does exert gravitational force. Because it did not interact with photons, dark matter was able to begin clumping due to gravity even before the ordinary matter (protons, electrons, etc.) had decoupled from radiation at the time of the CMB. These overdense regions of dark matter formed invisible gravitational wells, often called dark matter halos. After the universe became transparent and ordinary matter recombined, this newly formed neutral gas began to fall into the existing dark matter halos, drawn in by their enhanced gravitational pull. As more and more gas accumulated in these overdense regions, it became denser and hotter, eventually collapsing under its own gravity. Within these collapsing gas clouds, the first stars, known as Population III stars, ignited. As more stars formed and gas continued to accrete, these early stellar systems coalesced to form the first galaxies. These nascent galaxies continued to attract surrounding matter and merge with other small galaxies, growing larger over billions of years. Furthermore, gravity continued to pull these developing galaxies and their surrounding dark matter halos together into even larger structures. This process led to the formation of galaxy clusters, which are vast collections of hundreds to thousands of galaxies bound together by gravity. These clusters, in turn, are interconnected by filaments of galaxies and dark matter, separated by enormous voids, forming the large-scale structure of the universe known as the cosmic web. Therefore, the tiny hot and cold spots in the CMB represent the initial density variations that served as the gravitational seeds for the formation of all subsequent cosmic structures, from the first stars and galaxies to the immense galaxy clusters and the entire cosmic web.