Text 1 Dark matter is hypothesized to constitute approximately 85% of the universe's matter. Its existence is inferred from gravitational effects on visible matter, such as the rotation speeds of galaxies, which often exceed what visible mass alone can explain. Text 2 Alternative definitions of gravity, such as MOND (Modified Newtonian Dynamics), propose that at low accelerations, gravity behaves differently than Newton or Einstein predicted. These theories attempt to explain galactic rotation curves without invoking unseen mass.