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cold dark matter
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slow-moving massive particles, not yet identified, that don’t absorb, emit, or reflect light or other electromagnetic radiation
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cosmological principle
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the assumption that, on the large scale, the universe at any given time is the same everywhere—isotropic and homogeneous
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dark energy
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an energy that is causing the expansion of the universe to accelerate; the source of this energy is not yet understood
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evolution (of galaxies)
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changes in individual galaxies over cosmic time, inferred by observing snapshots of many different galaxies at different times in their lives
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galactic cannibalism
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a process by which a larger galaxy strips material from or completely swallows a smaller one
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homogeneous
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having a consistent and even distribution of matter that is the same everywhere
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hot dark matter
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massive particles, not yet identified, that don’t absorb, emit, or reflect light or other electromagnetic radiation; hot dark matter is faster-moving material than cold dark matter
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isotropic
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the same in all directions
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Local Group
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a small cluster of galaxies to which our Galaxy belongs
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merger
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a collision between galaxies (of roughly comparable size) that combine to form a single new structure
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starburst
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a galaxy or merger of multiple galaxies that turns gas into stars much faster than usual
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supercluster
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a large region of space (more than 100 million light-years across) where groups and clusters of galaxies are more concentrated; a cluster of clusters of galaxies
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void
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a region between clusters and superclusters of galaxies that appears relatively empty of galaxies