Gravity is due to the warping of space-time. Unlike the strong, weak and electro magnetic forces

(whos nature is unknown), gravity it cause by the shape of space, so why is it still refered to as

one of the forces along with the others?

It is not all clear that gravity is due to space-time warp. In fact, evidence indicates that the space-time warp is due to gravity...cause and effect-wise just the opposite of what

you assert.

In which case, a messenger particle (called the graviton) is a candidate for carrying that attraction message between masses. By example, the photon is the messenger

particle for EM forces and the gluon is that messenger for the strong atomic forces.

The whole field of string/M theory was developed in large part to integrate gravity into the same equations that describe the strong and weak atomic forces, and EM

force...the "others" in your question. Quantum gravity, another theory, has also developed because of the belief that gravity forces are carried by quanta; they are not caused

by space warping.

Unlike the photon and gluon messenger particles, the graviton has not yet been observed. However, with the Large Hadron Collider coming on line soon, physicists hope to

rectify that shortcoming.

The force due to gravity is a fundamental force - since all known objects have mass, they all possess gravity. The force of attraction between two objects with mass is due to gravity. Everything in this universe which is considered a true particle has a mass - hence all objects experience a force of attraction, which can only be due to gravity.

Gravity cannot be broken down into any other components (unlike forces such as friction), hence it must be fundamental. It causes motion - that is to say it causes objects to move (objects to attract eachother mutually) hence there must be a force present.