In quantum physics, which of these theorised sub-atomic particles has yet to be observed?

The theorized subatomic particle called the graviton has yet to be observed in quantum physics. The reason behind this is that gravitational forces are incredibly weak compared to the other fundamental forces of nature, such as electromagnetism. Hence, detecting and measuring the behavior of gravitons, which are the hypothetical force carriers of gravity, poses significant experimental challenges. While the existence of gravitons is predicted in the framework of quantum theory, their direct observation or experimental verification is yet to be achieved.