Although three of the four (known) fundamental forces have been unified by one paradigm called the Standard Model, Gravity remains outside.
Like General Relativity, Gravity appears, so far, not to fit. 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. ( 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, goto BBC In Out Time)
Other unanswered questions about Gravity :
• Why is so weak compared to the other forces (1040 times weaker than the electromagnetic force).
• Why is there apparently no 'AntiGravity'
See: Gravity in Wikipedia
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