In quantum field theory (QFT), the "most real" thing is often referred to as the vacuum state or the quantum vacuum. The vacuum state is the lowest energy state of a quantum field and represents the state of the field in the absence of particles.
However, it's important to note that the quantum vacuum is not empty in the conventional sense. According to quantum field theory, even in the absence of particles, the vacuum is teeming with quantum fluctuations. These fluctuations give rise to virtual particles, which continuously pop in and out of existence.
These virtual particles are not directly observable, but they have measurable effects. For example, the Casimir effect is a phenomenon in which two uncharged conducting plates experience an attractive force due to the presence of virtual particles in the vacuum. This effect has been experimentally verified and provides evidence for the reality of quantum fluctuations in the vacuum.
So, while the vacuum state is often considered the "most real" thing according to QFT, it is a quantum state characterized by fluctuations and the potential for particle creation and annihilation.