The image of the early universe that we see in the CMB reveals a time shrouded in darkness. During this era, the 17 known particles first started to combine to form the universe’s first atoms. Electrons, protons, and neutrons came together to create neutral hydrogen atoms, a process that allowed photons—or light—to traverse the cosmos freely. That is the glow we see in the CMB. Soon after this, well, cosmically soonish, gravity played its part and brought these atoms together, forming the first stars and galaxies. This “let-there-be-light” event occurred about 100 million years after the Big Bang.