Gravity!
This is the reason you are not floating around right now (unless you happen to be in outer space, but I assume you're not); the Earth's mass is producing gravity that holds you to the ground.
The Newtonian formula for gravitation isn't quite accurate (it has been replaced by Einstein's field equations in General Relativity), but it's actually extremely close as long as you're not near a black hole or traveling close to the speed of light.
Newton's gravitational equation says that the force F between two masses m1 and m2 is equal to the product of their masses, divided by the square of the distance r between them, all multiplied by a constant G to make the units work out:
`F = G {m_1 m_2}/{r^2}`
As far as the actual mechanism of gravity, that's actually still unknown. Many physicists hypothesize that there is an as-yet undiscovered particle that carries the force of gravity, a graviton, analogous to the particles that carry the force of electromagnetism, photons. But we've never found these gravitons, and we still don't know why it is that gravitational force is proportional to mass and not some independent charge the way that all other forces are.
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