For practical purposes- gravity is directly related to the mass of an object. The equation for the force of gravity is Gm₁m₂/r².

G is a constant that we don’t need to worry about for now. m₁ and m₂ are the masses of object 1 and 2, and r is the distance between the objects. So the force of gravity is directly proportional to the mass of each object- that is why the gravity of earth is so strong- the earth is massive! Technically everything with mass exerts a gravitational force. Say you are standing next to your friend- there is a very small gravitational force between the two of you, but since your friend’s mass is much less than that of earth, the gravitational force of earth is the only one you notice.

As for the exact origin of this force- that is a topic still being researched by scientists, but two theories were presented by Isaac Newton and Albert Einstein. Isaac Newton was the one to originally discover the force of gravity between two objects depends on the mass and distance between them, as described above. For most purposes, this definition works, but it breaks down when the objects approach the light speed.This is where Albert Einstein’s definition comes in.

Albert Einstein says that massive objects standing in space curve the space around them - thus attracting other objects. This can be conceptualized by imagining a trampoline, bowling ball, and marbles. Say you place a bunch of marbles on an undisturbed trampoline- the marbles will just sit in one place. Now say you gently place a bowling ball in the center- the trampoline will now sink/sag in the center where the bowling ball is, and all the marbles will roll towards the center. This is analogous to Einstein’s theory of gravity- massive objects “bend space” creating a well that causes other objects to be attracted.

Additionally, some scientists think gravity is caused by a particle called the “graviton”, but this particle is theoretical and has not been rigorously found/proved.