The early universe was uniform: the density of energy was everywhere almost the same. But it was not exactly the same. In quantum mechanics, quantities such as position, velocity, and energy density do not have exact values. Instead, their values fluctuate. We can describe their probable values—the most likely location of a particle, for example—but we cannot claim perfect certainty. Because of these quantum fluctuations, some regions of the early universe were ever so slightly more dense than other regions. As time passed, the attractive force of gravity caused more matter to move toward
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