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Scientist do not know what dark matter really is, but know more about what it is not. Only about 5% of the universe is made up of baryonic matter, or matter made of protons, nutrons, and electrons. 70% seems to be dark energy, and 20% dark matter. Two proposed theories for dark matter are that it is not made of normal particles, or that simply the laws of gravity on the scale of galaxies are different that those on a planetary scale. Simply put, scientist still do not know.