Dark Matter is believed to be responsible for 85% of gravity in the universe, and no one knows what it is or how it works.
According to Neil deGrasse Tyson, Dark Matter is the longest-standing unsolved mystery in modern astrophysics. Indeed, it might not even be matter! Basically, the amount of gravity in the universe doesn't quite equal the amount of observable mass – the planets, the stars, the galaxies, the comets, the black holes, and the dark clouds. So scientists propose there is a large measure of unobservable or "dark" mass in the universe, which is the source of all that gravity.
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