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The universe still has a number of mysteries, but the ones that most vex physicists are dark matter and dark energy . This type of matter and energy is detected by its gravitational influences, but can't be observed directly, so physicists are still trying to figure out what they are. Still, some physicists have proposed alternative explanations for these gravitational influences, which do not require new forms of matter and energy, but these alternatives are unpopular to most physicists.
