Gravity is the force that attracts objects with mass towards each other. On Earth, it is what brings us back to the ground if we jump upwards. In space, it is what makes the Moon orbit the Earth, what makes the Earth and other planets orbit the Sun, and what makes the Sun and stars orbit the centre of the galaxy. According to physicists, there are only four forces in the universe, and gravity is the weakest by a ridiculously huge amount. But two of those forces have extremely limited range, and the third (electromagnetism) has both positive and negative forms which tend to cancel each other out over long distances. Only gravity always adds up and extends out to infinity, which is why it is the dominant force despite being so much weaker.
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