Ordinary matter, also called baryonic matter, consists of protons, neutrons, and electrons. This is the material that emits or absorbs radiation, allowing telescopes to observe stars, nebulae, and galaxies. From the largest galaxy cluster to the smallest grain of dust, everything visible belongs to this category. Yet despite its familiarity, this matter represents only a sliver of the cosmos. It is the island of visibility on which all human understanding of the universe has so far been built.
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