Hundreds of years of observation have established the existence of a universal attraction between physical objects. In 1687, Isaac Newton quantified this phenomenon in his law of gravity, which states that every object in the Universe attracts every other object, with a force between any two bodies that is proportional to the product of their masses and inversely proportional to the square of the distance between them.
Gravity is actually an extremely weak force.
Nevertheless, gravity is the dominant force on the large scales of interest in astronomy. There are two reasons for this. Second, gravity is additive. Planets and stars are very nearly electrically neutral, so the forces exerted by positive and negative charges tend to cancel out. As far as we know, however, there is no such thing as negative mass, and no such cancellation of gravitational attraction.