Gravity is defined as "an acceleration of mass." What causes gravity?

Matter has mass and everything with mass has gravity.

Shortly, mass cause gravity.

interesting question every matter in the universe has mass or weight when enough neutrons and protons form it becomes heavier now in order to accelerate mass you need energy mass can never exceed light as its mass would be infinite and would take infinite amount of energy to do so however alcuberie drive aka warp drive could bend time and space now what causes gravity is a hypothetical particle called gravitons which has yet to be observed there are several planned experiemnts in the future to detects these its assumed graviton is actually a higgs or boson particle which is what give matter its mass so the answer is 99% mostly like gravitons which has yet to be confirmed by scientists!.

The attraction for two bodies is directly proportional to the mass and varies inversely with the distance between them, cause (as far as i know) still unknown.

According to the law of gravitational

" Gravitation between two bodies is directly proportional to the mass of the body and inversely proportional to the square of the distance between these two bodies"

Gravitation = mass1 X mass2 /square of the distance