Some flat-Earthers mentioned the fact that graviton —the mediating particle of gravitation— is ‘not yet observed’. They took this fact as ‘proof’ that gravity does not exist.
Often they would triumphantly show us screenshots of the table of fundamental particles they took from Wikipedia and other sources. In the ‘gravitation’ column, they would point out its mediating particle has not been observed yet. And they would conclude that scientists are aware gravity does not exist, and we all have been lied to.
Obviously, they are wrong on all accounts.
Gravity is a natural phenomenon that has been observed. It is real, can be measured and can be felt.
Graviton is a hypothesized fundamental particle that mediates the force of gravity within the framework of the Theory of Quantum Mechanics. Graviton has not been successfully observed yet, and gravity has not been successfully explained within the principles of Quantum Mechanics.
Quantum Mechanics already explains all the three fundamental forces other than gravity. Currently, gravity has to be explained separately using the Theory of General Relativity, which is presently most accurate explanation of gravity.
In time, graviton might be discovered, and it would pave the way to the unification of both theories so that all the fundamental interactions that define the universe can be explained in a single, unified theory.
The same thing happened to all the other mediating particles. For example, Photon is the mediating particle of the electromagnetic interaction and was discovered only in 1927. It doesn’t mean before 1927 we didn’t know the existence of light and electromagnetism. The fact that graviton is not yet observed does not disprove gravity.