RICHARD KAIL / SCIENCE PHOTO LIBRARY RICHARD KAIL / SCIENCE PHOTO LIBRARY
Dark matter, conceptual image. Computer illustration representing the presence of dark matter in the Universe. Dark matter, which holds clusters of galaxies together, accounts for nearly one-quarter of the mass of the universe, but its exact nature is not known. It is virtually invisible to current scientific tools and its existence is surmised by looking at the way gravity is warped far beyond what can be attributed to visible matter.
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