Gravity - a

force created by the presence of

mass.

Any two pieces of mass attract each other with a force more or less

inversely proportional to the square of their separation and

directly proportional to the product of their masses. This is

Isaac Newton's famous law of gravity,
F = Gm

_{1}m

_{2}/r

^{2} where G is the gravitational constant.

Of course, this has been proven wrong by

Albert Einstein's theory of

general relativity, which treats gravity as the result of the warping of space-time by mass. In most cases, the answers given by relativity match those of Newton's law.
Gravity is always attractive. Two pieces of mass are never repulsed from each other due to gravity.

Black holes are one prediction of general relativity, as are

gravity waves and various other phenomena.

General relativity isn't the final word on

gravity, either. Attempts at combining GR with

quantum mechanics and the unification of the four forces (

electromagnetism,

gravity, the

strong nuclear force, and the

weak nuclear force) under one

unified theory have lead to things such as

superstring theory and

M-theory which describe the universe in ways different from both QM and GR.