Although three of the four (known) fundamental forces have been unified by one paradigm called the Standard Model (SM) Gravity remains outside.

This doesn't necessarily mean that Standard Model has failed in some way. Perhaps Gravity is independent from and incompatible with the other three forces. ( viz. the Electromagnetic force, the Strong force and the Weak force.)

Unlike the other three forces, which have all had their force-transmitting particles detected and measured by experiment, the Graviton has not yet been detected - and theory suggests that it would probably require a particle accelerator the size of the solar system to do so. (For a 45 min. podcast discussion about the graviton, listen to the BBC's In Out Time programme on the subject)

Other unanswered questions about Gravity :
• Why is it so weak compared to the other forces - some 10⁴⁰ times weaker than the electromagnetic force?
• Why is there apparently no 'AntiGravity'?

See: Gravity in Wikipedia

Also see The Gravitational constantplugin-autotooltip__plain plugin-autotooltip_bigThe Gravitational constant

The gravitational attraction between two objects is dependent on the mass of the objects, the distance between them, and the gravitational constant (G).

The masses and distance can vary, but the constant, as the name implies, is always a fixed number.

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